This also happened to me. A few patches ago blizzard had removed the old patch-2.mpq from the data folder. Last patch they added it back in as an official mpq. Before this patch I had a model edit named patch-2.mpq so when the patch was applied it assumed MY patch-2.mpq was the official patch-2.mpq being re-implemented back into the game. So the game would run the mpq without the MEF. This is still possible if you replace the officail patch-2.mpq with your model edit. But only one model edit will be possible at a time without using the MEF. Hope this helps clear things up!