Thank you for your help Miikerman!
I forgot to mention that I have tried using a different micro-SD card too, and the problem was not solved. I have now re-installed the firmware and it would seem that the problem has been fixed, at least a short test would indicate so. Lets hope that this solved the problem and there is no need to send the player back to Sandisk.
PS: I did not manage to reset the player, since holding the power button did not do anything, but I would suspect that the re-installing of the latest firmware did the same allready?
