Yes, MIkeLupe, that’s right. I forgot that.
In my case it worked on version 13 quite long, by increasing the max-version in the info.xml. The reason, why the developer decided not to put on version 13 is a bug, which describes my issue.
I don’t know why nextcloud was able to open the kdbx files with keeweb on my old server. Seems to be luck. Maybe a file scan on the new server broke the mime type adjustment.
Here is the bug: https://github.com/jhass/nextcloud-keeweb/issues/34
Its very sad. The combination of nextcloud, keepass and keeweb was very usefully.