m3u playlists just work fine. But if the playlists refers to wma files instead of mp3, it doesn't work - there's no playlist built. I tried the extensions m3u, pls or txt within a directory watched by inotify, but without success. Hitting the + in the web frontend doesn't solve the problem.
I can play the wma directly via "PC Directory".
problem solved. The program I built my playlist with (don't be sure which one I used) put "\" instead of "/" in the file's path - maybe it "thought" I would use windows because of the wma files, which is wrong.
After I changed backslashes to slashes everything works fine, regardless if I point to wma or mp3 files.