The issue with empty notes has been started several times in the past. My latest post was here: https://support.nimbusweb.co/portal/en/community/topic/server-backup-not-usable

Now I found an explanation for some of the notes with missing text:

When I try to open such a note in Android, I get the following message:

Means: You have stopped supporting notes which has been created with an old editor. Great! The general purpose of a note-taking application is to take notes and can read them in the future!