This has nothing to do with Plasma 6 move, so please keep on topic, this time post moved for you to appropriate tags.
Appimages are not supported by KaOS (Flatpaks has been made the supported choice for bundled packages), plus the best way to add missing packages is still building yourself:
https://kaosx.us/#repo
https://kaosx.us/docs/package

But it might be someone will answer here about appimages.

6 days later
  • [deleted]

You do have Appimagelauncher installed, right? If not install it and if you still have issues I suggest you ask the creator, cause most issues that crop up on a system either are hardware or installed apps and not the OS no matter which OS it is.

    25 days later

    [deleted] Hello, sorry for the late reply. I didn't noticed it.

    On the Live system I get the "text busy" error, but when I install it the AppImages work well. I'm using Nextcloud, LibreOffice and Cryptomator Appimages without any issues