Usually when a player does not read a file it is because it is the wrong format but judging from the firmware version it seems to be an AMV format player. The demo you deleted was it just on the player or in a folder. Some players look for files in specifically named folders like AMV, Video or Movies.
You could also try formating the player as there may be errors in the memory.
Try some of these samples and see if they work:
http://www.mympxplayer.org/sample-videos-dc7.html