I had this problem when trying to play a file which was 48kHz instead of 44.1kHz. I used Audacity to convert it, after which it was fine. See page 12 of the owner's manual.
Just checked the specifications section of the manual and it says
Playback format
WAV: 44.1kHz, 16/24 bit
MP3: 44.1/48kHz, 32/64/96/128/192/256/320kbps
I'm not sure about the MP3 though, but I can't remember if it was WAV or MP3 I had problems with