I'm running Mac OS X version 10.6.8
Did any new plugins get installed with 12? Different pref files for 11 and 12 so shouldn't be a conflict there.
Address Book Selector.fmplugin
Ony 1 of those I have active is mailit - v5.
I would check the extensions folder within the FileMaker Pro 11 application folder and the Extensions Folder in the user/Library/Application Support/FileMaker folder and look for any duplicates or conflicts.
Start by removing them all and see if FMP11 launches. If so gives you a starting point