I "fixed" the frozen UI by rebooting the host. Later I got problems with the back up.
The fix with the changed file worked for me.

BTW: I have a new backup destination "Copy of Default". This backup gets stuck with the following error:
Error opening new cache file: ENOENT: no such file or directory, open '/home/yellowtent/platformdata/backup/71675862-9254-4cb1-91ce-4c387c56723f/0038cb4e-cf5a-42f5-aeca-c5fbe7e86bad.sync.cache.new'
Should I open a new ticket for this?