Have you tried converting the original 6.01 maya file to an .Obj file when saving instead of making it a .mb file? If you don't have the option to save it as an .Obj file go into
Windows > Settings/ Preferences > Plugin Manager

Under objExport.mll put a check mark beside loaded and auto load then close that window. Then now when you save the file you'll have the option to save it as an obj. I've been having similar problems opening my 6.01 on my ver 5. at home. The only problem with saving them as an .obj is maya will automatically combine your object. If you don't want your object one item, then jump into Edit Polygons > Seperate to seperate that item.

It might be a pain in the butt to reattach all the pieces you want, but thats the only solution I've found so far.