Without detailed information I may not be able to be really helpful there.

First possibility is, that the *.mmm file is corrupted. Did you ever try making a new *.mmm file and reload it?

Second possibility is that you try to open a file from a new version with a very old version. Actually the program should recognize the *.mmm and just tell that it can't be opened, but who knows.

So, where is the *.mmm from? Which version do you use? Do other *.mmm's work?