My guess:
Checkout the permissions/ownership of your playlists.
Try to change to 666 all of them. The directory should be 777.
I also think it is not a good idea to run mpd as root. This you can change in mpd.conf, if you set the "User". Make sure that all users belong to the same "default" group e.g. audio, if
you run a multiuser setup. If any user generates a file it'll get the same gruop id assigned. This is the way I got mpd working in a multiuser environment.
Foreach user I had to run "newgrp audio" to change the default group configuration.
I hope this helps.
Good luck.