$ mirror-check
 
Checking core main apps...
 
md5sum: /tmp/.1002mirrordb.tmp: No such file or directory
[core] is not synced.
Do not update, wait for your mirror to sync.
md5sum: /tmp/.1002mirrordb.tmp: No such file or directory
[main] is not synced.
Do not update, wait for your mirror to sync.
md5sum: /tmp/.1002mirrordb.tmp: No such file or directory
[apps] is not synced.
Do not update, wait for your mirror to sync.

I have just the same problem. Just same output when trying to update system.

That means the mirror you use is down (fosshost changed their mirror, and was the default on the February ISO), but without the mirrorlist, not clear which mirror you use.
Open your mirrorlist:

kate /etc/pacman.d/mirrorlist

and select a different mirror or edit the fosshost mirror to look like:
https://github.com/KaOSx/core/blob/master/pacman-mirrorlist/mirrorlist#L7

To see the status of the current mirrors:
https://kaosx.us/packages/mirrors.php