(T) Blue & Me: no media found

I got the error two times in one month now and I don't know what would be the cause of it... But I can listen to my sings when I shuffle it.

When I use the media player, I go to maps or artists,... then B&M keeps telling "no media found". Most of the time, I just choose an artist to listen to my music. But a few days ago, I switched back to shuffle, then yesterday I tried to choose an artist, but since then I have the error 'no media found'. I only can listen to my music when I choose to shuffle the songs.

When I format the USB-stick on my pc, then move the songs back, everything works fine, but probably again for just a few weeks...