What version is your firmware? We will need to know if it is an Action chipset or Rockchip player.
If it is Action, format errors generally occur when the player has been hacked to show more memory than what is really on the player. The player will show that the file has been downloaded, but in reality, it is a ghost file and when you try to play it comes up with format errors. If it turns out to be an Action chipset player, I suggest you follow the instructions posted here:
How To: Removing Memory Hack in order to find out the true memory and prevent further problems with your player.
If it is a Rockchip player, format errors are usually the result of using the wrong screen size when converting. So if your player is Rockchip, we will need to know the screen size in order to help.