Things which have caused this error for other users:
1) Incorrect DirectX version (or no DX)
2) Sound driver issues
It doesn't sound like these are the problem for you since you already had it working...check all the same tho.
One other thing you could try: edit the ini file for whatever emu/list is currently selected (eg mame-0.ini), and set 'current_game' to something different. It is a long shot but if your snapshot for the selected game is bad for some reason, it might cause this error when trying to decode the PNG.