No, that was the reason. 😉
It _could_ not add the song to the database, in fact it could not add anything to the database because it could not write to the database. 😉
Maybe the file permissions for songs.db are haywire too. You shouldn’t start firefly from the shell with totally different users.
sudo to the user that firefly is normally running as, which would be nobody, I guess? Again, this info can be found in the mt-daapd.conf.