Not the .db file, but the directory it’s sitting in (/opt/var/mt-daapd).
If the “runas” user can’t create files in /opt/var/mt-daapd, that’s when you get the error described.
So check to see that /opt/var/mt-daapd is owned by the user that is the “runas”, and delete the songs.db file. Then start it and see where you get.
p.s. you can check this interactively by doing “su guest”, then changing to /opt/var/mt-daapd and see if you can create a file.
slug:/# su guest
slug:/$ cd /opt/var/mt-daapd
slug:/opt/var/mt-daapd$ touch test.fil
if that won’t work, then mt-daapd won’t work either.