Ok, this is probably a permission problem.
On Ubuntu runs as user nobody. Make sure that user has access to.)
- the directory where the plugins reside
the directory where the DB resides
the web_root directory for firefly
the directory where the music files are at
You can chown the former 2 directories to that user, while you probably have to take more care about the permissions to the music directory.
To find out all these paths, check your config file /etc/mt-daapd.conf