Dear Huib,
Any ideas on what can cause such a message? When was the last time the format did change? What can we do about it (other than do as it says, assuming it's not actually a feature)?
Hm, the database version hasn't been changed since Version 5.91 (which was released more than 18 months ago):
https://github.com/BaseXdb/basex/blob/master/src/main/java/org/basex/data/Da...
What happens if you try to open your database with the latest official version of BaseX? Have you done backups on your database, which you could try? If the problem persists, feel free to send me the inf.basex file of your database, which might give us some hints.
Best, Christian