@kugiigi I deleted (with a backup first) ~/.cache/mediascanner-2.0/mediastore.db and then, after restart, I had a smaller .db to look into and there I saw two files in the ~/Videos folder and one of them was a .aac file - an audio file. As soon as I moved it in the ~/Music folder the Music app started working again (playing tracks not added to a playlist).
Thank you again for all the support and pointing me in the right direction.
This was an interesting one!