Sounds nasty. Does it do the same with any .mp file?
The only thing I would suggest is making another copy of the whole LED to another location on your HD and see if it works. Don't copy the one you have, d/l a fresh one. Delete the original LED and also the .ini file which may be in the windows folder or it may be buried somewhere else - depends on your os. Do a search for .ini files. You could try deleting this file before installing a new copy of LED and see if that makes any difference.
If it's just one .mp then follow previous advice. I have some RAS .mp files and some of those will not open at all in the LED.
All the more reason to backup your .mp before every editing session.