I deleted the Mainstage plist file. Still getting a blank screen, but it did open up some more file menu items (All of File/Edit menus are now active "black instead of grey"), although they do not do anything still.
Then I tried to do a New Concert, and it started to load and crashed again, similar to last time.
I did reinstall Mainstage also, and that didn't help, but I'm not sure if I did remove all of the Mainstage related files. I used CleanApp, and it pulls all related files into a queue, but not sure if I deleted all of the proper files, because it shares a lot with Logic - I only deleted files that had "mainstage" in the name.
I'm thinking about reinstalling everything again, but that is not my favorite idea. I have to install 7 first, then the upgrade again (w/ content discs). It already took a few hours.
Any other tips?
By the way, thanks for posting that possible solution.
I do have an appointment with a Genius tomorrow, but not sure if they'll be able to fix it either.