I had a little look at this again with the Akonadi Console to see what messages showed up when adding the caldav resource, and it does indeed seem to be struggling with something.
For every entry in the calendar I get this message:
[2020-01-28T08:35:58.563Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/1C38C107-E12C-46B1-BB05-EABCED662BAD.ics"
I tried downloading just that file and it seems to have all the info it should, importing it into kOrganizer works just fine and the event shows up. So I am not sure what Akonadi is having trouble with..
There is also an Error happening but I don't know what for.
Here is the full log (fetching one homeset with three calendars):
[2020-01-28T08:35:51.818Z] akonadi_davgroupware_resource org.kde.pim.kdav Found 1 homesets
[2020-01-28T08:35:51.818Z] akonadi_davgroupware_resource org.kde.pim.kdav ("https://p25-caldav.icloud.com:443/<icloud id>/calendars/")
[2020-01-28T08:35:52.338Z] akonadi_davgroupware_resource org.kde.pim.kdav "https://p25-caldav.icloud.com:443/<icloud id>/calendars/f1d70f9d-0236-4fc5-a296-f0a8bcb664ab/" PRIVS: QFlags(0x1|0x2|0x4|0x8|0x40|0x100|0x200)
[2020-01-28T08:35:52.338Z] akonadi_davgroupware_resource org.kde.pim.kdav "https://p25-caldav.icloud.com:443/<icloud id>/calendars/home/" PRIVS: QFlags(0x1|0x2|0x4|0x8|0x40|0x100|0x200)
[2020-01-28T08:35:52.338Z] akonadi_davgroupware_resource org.kde.pim.kdav "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/" PRIVS: QFlags(0x1|0x2|0x4|0x8|0x40|0x100|0x200)
[2020-01-28T08:35:53.878Z] akonadi_davgroupware_resource org.kde.pim.davresource Retrieving collections list
[2020-01-28T08:35:54.982Z] akonadi_davgroupware_resource org.kde.pim.kdav Found 1 homesets
[2020-01-28T08:35:54.982Z] akonadi_davgroupware_resource org.kde.pim.kdav ("https://p25-caldav.icloud.com:443/<icloud id>/calendars/")
[2020-01-28T08:35:55.511Z] akonadi_davgroupware_resource org.kde.pim.kdav "https://p25-caldav.icloud.com:443/<icloud id>/calendars/f1d70f9d-0236-4fc5-a296-f0a8bcb664ab/" PRIVS: QFlags(0x1|0x2|0x4|0x8|0x40|0x100|0x200)
[2020-01-28T08:35:55.512Z] akonadi_davgroupware_resource org.kde.pim.kdav "https://p25-caldav.icloud.com:443/<icloud id>/calendars/home/" PRIVS: QFlags(0x1|0x2|0x4|0x8|0x40|0x100|0x200)
[2020-01-28T08:35:55.512Z] akonadi_davgroupware_resource org.kde.pim.kdav "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/" PRIVS: QFlags(0x1|0x2|0x4|0x8|0x40|0x100|0x200)
[2020-01-28T08:35:55.513Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onRetrieveCollectionsFinished: Collection disappeared. "https://p25-caldav.icloud.com/<icloud id>/calendars/home/"
[2020-01-28T08:35:55.513Z] akonadi_davgroupware_resource org.kde.pim.akonadicore localListDone: false deliveryDone: true
[2020-01-28T08:35:55.514Z] akonadiserver org.kde.pim.akonadiserver Error while handling command FetchItems on connection akonadi_davgroupware_resource_3 (0x11e8ab0)
[2020-01-28T08:35:55.514Z] akonadiserver org.kde.pim.akonadiserver Error while handling command FetchItems on connection akonadi_davgroupware_resource_3 (0x11e8ab0)
[2020-01-28T08:35:55.515Z] akonadiserver org.kde.pim.akonadiserver Error while handling command FetchItems on connection akonadi_davgroupware_resource_3 (0x11e8ab0)
[2020-01-28T08:35:55.515Z] akonadiserver org.kde.pim.akonadiserver Error while handling command FetchItems on connection akonadi_davgroupware_resource_3 (0x11e8ab0)
[2020-01-28T08:35:55.516Z] akonadiserver org.kde.pim.akonadiserver Error while handling command FetchItems on connection akonadi_davgroupware_resource_3 (0x11e8ab0)
[2020-01-28T08:35:55.517Z] akonadiserver org.kde.pim.akonadiserver Error while handling command FetchItems on connection akonadi_davgroupware_resource_3 (0x11e8ab0)
[2020-01-28T08:35:55.518Z] akonadi_davgroupware_resource org.kde.pim.akonadicore localListDone: true deliveryDone: true
[2020-01-28T08:35:55.539Z] akonadi_davgroupware_resource org.kde.pim.akonadiagentbase Preparing collection sync of collection 34 "リマインダー ⚠️"
[2020-01-28T08:35:55.541Z] akonadi_davgroupware_resource org.kde.pim.davresource Retrieving items for collection "https://p25-caldav.icloud.com:443/<icloud id>/calendars/f1d70f9d-0236-4fc5-a296-f0a8bcb664ab/"
[2020-01-28T08:35:55.656Z] akonadi_indexing_agent org.kde.pim.akonadi_indexer_agent indexed 34
[2020-01-28T08:35:55.781Z] akonadi_indexing_agent org.kde.pim.akonadi_indexer_agent indexed 35
[2020-01-28T08:35:55.856Z] akonadi_indexing_agent org.kde.pim.akonadi_indexer_agent indexed 36
[2020-01-28T08:35:56.221Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/f1d70f9d-0236-4fc5-a296-f0a8bcb664ab/5af1551a-3d61-4856-abaa-27586f725249.ics"
[2020-01-28T08:35:56.221Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/f1d70f9d-0236-4fc5-a296-f0a8bcb664ab/f15a7615-e80a-4aef-9e33-6c1ac36adf82.ics"
[2020-01-28T08:35:56.222Z] akonadi_davgroupware_resource org.kde.pim.davresource Updating collection CTag from "" to "FT=-@RU=85a3c40c-06ba-4ade-8df4-d6d98832e2bc@S=11"
[2020-01-28T08:35:56.222Z] akonadi_davgroupware_resource org.kde.pim.akonadicore Received: 0 Removed: 0 In total: 0 Wanted: -1
[2020-01-28T08:35:56.222Z] akonadi_davgroupware_resource org.kde.pim.akonadicore ItemSync of collection 34 finished
[2020-01-28T08:35:56.223Z] akonadi_davgroupware_resource org.kde.pim.akonadiagentbase Preparing collection sync of collection 35 "自宅"
[2020-01-28T08:35:56.228Z] akonadi_davgroupware_resource org.kde.pim.davresource Retrieving items for collection "https://p25-caldav.icloud.com:443/<icloud id>/calendars/home/"
[2020-01-28T08:35:56.381Z] akonadi_indexing_agent org.kde.pim.akonadi_indexer_agent indexed 34
[2020-01-28T08:35:57.331Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/home/1AFA798F-0857-4C06-9762-45F15D840F4D.ics"
[2020-01-28T08:35:57.331Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/home/7C0CB614-21D4-4CA4-8FC7-D8C2314E5A5B.ics"
[2020-01-28T08:35:57.332Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/home/8F08DA41-9ADF-4DAB-83A2-486AB8ADA52E.ics"
[2020-01-28T08:35:57.332Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/home/A8D94629-4CDA-4F27-8FD0-11A6D1F04FAA.ics"
[2020-01-28T08:35:57.332Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/home/AB58D096-7F1E-4A78-B825-E7BE1831E650.ics"
[2020-01-28T08:35:57.332Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/home/BF873FBD-8438-43A4-9DB2-4E69048BB949.ics"
[2020-01-28T08:35:57.333Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/home/EF2DDE61-B41E-42A3-8BD1-CB53D4651D1E.ics"
[2020-01-28T08:35:57.333Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/home/F1CC42C8-28F1-458B-907C-C11A17F8990F.ics"
[2020-01-28T08:35:57.333Z] akonadi_davgroupware_resource org.kde.pim.davresource Updating collection CTag from "" to "FT=-@RU=85a3c40c-06ba-4ade-8df4-d6d98832e2bc@S=116"
[2020-01-28T08:35:57.333Z] akonadi_davgroupware_resource org.kde.pim.akonadicore Received: 0 Removed: 0 In total: 0 Wanted: -1
[2020-01-28T08:35:57.334Z] akonadi_davgroupware_resource org.kde.pim.akonadicore ItemSync of collection 35 finished
[2020-01-28T08:35:57.335Z] akonadi_davgroupware_resource org.kde.pim.akonadiagentbase Preparing collection sync of collection 36 "職場"
[2020-01-28T08:35:57.339Z] akonadi_davgroupware_resource org.kde.pim.davresource Retrieving items for collection "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/"
[2020-01-28T08:35:57.465Z] akonadi_indexing_agent org.kde.pim.akonadi_indexer_agent indexed 35
[2020-01-28T08:35:58.563Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/1C38C107-E12C-46B1-BB05-EABCED662BAD.ics"
[2020-01-28T08:35:58.563Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/32C502E5-35FD-406C-A532-3DCE833EEE30.ics"
[2020-01-28T08:35:58.564Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/49CBF875-288C-451C-AFB5-859129CA465F.ics"
[2020-01-28T08:35:58.564Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/4C1B0BA4-13EA-41E5-91DE-EA436F6A0385.ics"
[2020-01-28T08:35:58.564Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/4DCD6B9F-CA00-4A2A-B92F-F57786371F5C.ics"
[2020-01-28T08:35:58.565Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/4E8767BD-ABA6-41B1-878A-D03222C3C24F.ics"
[2020-01-28T08:35:58.565Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/5E10B4CE-BC30-4E5C-885C-ED1DDF6EBFD4.ics"
[2020-01-28T08:35:58.565Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/613314A3-E14A-4BB2-9EE3-591D07CF91D6.ics"
[2020-01-28T08:35:58.565Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/63EBC54C-08F1-4775-AD86-097802CC0928.ics"
[2020-01-28T08:35:58.566Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/69D99BD3-6ACC-4E8D-B14B-94926869E0A2.ics"
[2020-01-28T08:35:58.566Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/6DE87B5D-F461-42E2-A43C-87C0467C1D9E.ics"
[2020-01-28T08:35:58.566Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/79F42E5D-DC17-4499-B27B-F2F4666123B9.ics"
[2020-01-28T08:35:58.566Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/7C364C4D-FAFF-4E2E-A68C-73F6E61E0B0A.ics"
[2020-01-28T08:35:58.566Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/86CC5DB8-E2F9-48BF-AC0B-F1EA947F2584.ics"
[2020-01-28T08:35:58.567Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/AF31F6A5-559A-434D-A8A9-74079263E731.ics"
[2020-01-28T08:35:58.567Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/B1749142-74E5-4567-ADBD-9125E717B1CB.ics"
[2020-01-28T08:35:58.567Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/BDED529C-1DDA-44E0-96E4-524D7B700109.ics"
[2020-01-28T08:35:58.568Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/BE091904-7A0F-4718-85F7-1F4965EFD297.ics"
[2020-01-28T08:35:58.568Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/C1FE26EA-DF42-42DC-A056-045DC8980D00.ics"
[2020-01-28T08:35:58.568Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/C844FC5F-FBE9-4529-B1EB-DE2DEEC24E09.ics"
[2020-01-28T08:35:58.568Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/CE013A6D-0D48-48C9-8F21-398915E35161.ics"
[2020-01-28T08:35:58.568Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/D3A4324A-E421-4F45-B98F-137D09FDAB79.ics"
[2020-01-28T08:35:58.568Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/E30ED019-9586-46DB-A40A-E60484B1EA26.ics"
[2020-01-28T08:35:58.568Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/EB9C7F6E-14EA-4DD5-8FC6-4D6EC99E8BDE.ics"
[2020-01-28T08:35:58.568Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/EC419278-1C53-47DB-8D0F-245F35BE065A.ics"
[2020-01-28T08:35:58.569Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/EF1B05BB-36C3-47E3-8B4E-E3B88263B797.ics"
[2020-01-28T08:35:58.569Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/F0FA4D0E-7F02-473C-B9E8-0F0B129061DF.ics"
[2020-01-28T08:35:58.569Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/F35EF58D-806B-4CA6-A71A-39212E391511.ics"
[2020-01-28T08:35:58.569Z] akonadi_davgroupware_resource org.kde.pim.davresource DavGroupwareResource::onMultigetFinished: Empty item returned. "https://p25-caldav.icloud.com:443/<icloud id>/calendars/work/F3BC653B-FF9D-4BF0-B404-F44E7DABB7D5.ics"
[2020-01-28T08:35:58.569Z] akonadi_davgroupware_resource org.kde.pim.davresource Updating collection CTag from "" to "FT=-@RU=85a3c40c-06ba-4ade-8df4-d6d98832e2bc@S=132"
[2020-01-28T08:35:58.569Z] akonadi_davgroupware_resource org.kde.pim.akonadicore Received: 0 Removed: 0 In total: 0 Wanted: -1
[2020-01-28T08:35:58.569Z] akonadi_davgroupware_resource org.kde.pim.akonadicore ItemSync of collection 36 finished
[2020-01-28T08:35:58.699Z] akonadi_indexing_agent org.kde.pim.akonadi_indexer_agent indexed 36