You are windows, right? Try COMPLETELY disabling UAC. Google will be your friend at this. UAC causes huge problems with letting the computer access needed files and there's no good error reporting for this. Also, if UAC is on, the map will end up cached in your Appdata, so if there was a problem once and you fixed it, it will still be using the old version.
On another hand, MGM files do not work in GAE yet (not yet young grasshopper, not yet), so it must be regular gbm format. However, it may be worth trying the original map editor. I've experienced no problems with Titi's map editor, but perhaps something is wrong with your computer. Also, have you made sure your drivers are up to date (don't see why that would be the problem, but hey, you never know...)