I found these 2 topics with the same error. The problem is your run_as user in your config file.
It looks like you’re running as root? Baaad idea. I suggest you use the daemon to run mt-daapd: /etc/init.d/mt-daapd start
This way the run_as user from the config will be used for sure.
Next you need to make sure is you’re running mt-daapd as “nobody”.
Yes, nobody. So you need to make sure all music files can be read by anybody.