I tired to import new files and end up with the following error message:
beet import -i /home/matthias/Multimedia/Musik/flac/ error: database file /home/matthias/Multimedia/Musik/beets/beet_music_data.db could not be opened
Also an re-import of the already imported filed had the same error.
Even when I removed the existing database file (and I made of course a backup of it) and therefore force beets to create a new one showed the same error (the new database file has been created with the size 0).
I got the feeling that this correlates with the update from debian 9 to debian 10 I made a few weeks ago.