Moinmoin nach Hamburg. 🙂
Either your router doesn’t properly allow multicasts.
What router are you running? I suspect a WRT54 of some sort;)
What does the syslog say?
if mt-daapd is unable to acces the mp3_dir, it terminates.
For a test you could just specify an empty directory where the mt-daapd user has read permissions, so it would come up and you could access the web-interface.