Some users have reported that Audio Components can be creating such a problem when FCP does not start.
So I proceeded by elimination: pulling recently installed AUs from my Plug-Ins folder (in BootHD/Library/Audio/Plug-Ins/Components) to see if it helped.
Indeed I found my culprit: and audio component I installed a few weeks ago: "MIGIC.component." After removing it, FCP started no problem.
