what does the mt-daapd.conf say under runas user?
I guess that its nobody, or guest.
It looks like this user does not have access to the share, mp3_dir, and/or no access to the database location, which is defined under db_param.
to test this, open a terminal, and do:
ls -al /path/to/your/share
you’ll see if this user has access rights, or not.
The user mt-daapd runs as, must have write access to the DB folder, and at least read access to the mp3_dir.
In case this is all too much hazzle, and you just want to run the server in yout private, local network it probably is easier to just change the runas-user to your own user. Be aware though that anyone with access to mt-daapd can possibly hack your account and wipe all your data, or worse. 😉