Thank you all for the detailed answers. Until now I have always used the "Kobo Extended Driver", but the error with the database also appears when I use the other driver "Kobo Touch" - regardless of whether EPUB or KEPUB is used.
I also wrote about 2 damaged devices, one of which has now been replaced by the Kobo support. Unfortunately the database errors appears again

And yes, all documents have been checked positively. Many of them were automatically created by LATEX with output to EPUB.
I now see 3 options: 1) waiting for a new firmware release (as everybody else), 2) using NickelMenu (so far no experience on my side with that) and 3) permanently switching to Pocketbook. The latter option seems safest to me at the moment.
Personally, I'm surprised that the database error hasn't been fixed yet. As a ereader manufacturer, I would pay close attention to the discussions in the various MobileRead forums - because it is usually very difficult to get detailed feedback from different customers.
Thanks again everyone for the discussion here!