It means another instance of mt-daapd is already running. You will probably have to delete your songs.gdb, if you had multiple mt-daapds running on the same database. The possibility of database corruption is pretty high.
For some reason the signal handling on the nslu2 is different than any of the other platforms I’ve run mt-daapd on, and I can’t get it to stop cleanly (except via the web interface). I’ve always had to do a “killall -9 mt-daapd” on the nslu2. Which is hackish, but I don’t have a better way (except the web interface).