@unaegeli next time, just answer after you first try, not after twenty ones and two month. 😉

So, did you use UT tweak tool to remove cache and config data?

Maybe Ghostcloud only works for Nextcloud instances?

As you are mentioning UBsync, did you check logs? Try the the LogViewer App.

Maybe its an issue regarding server certificates?
You should use a public one like lets encrypt.

We could test your FP2 using my personal nextcloud instance. Let me know, if you want to investigate this any further.