I don’t think I’m necessarily going to be much help, but just a thought.
When you trash the preferences for FCP you lose any and all “customization” or personalization to FCP. If there were any settings that you are used to using, or haven’t been changed on your computer in a while, (you may not have realized you were working with them), this could be the issue. I’ve trashed my preferences a couple of times and it has worked cleanly, but I had to reset any personalization or customization I had previously worked with.
Again, probably not much help, but maybe this could clue you in to something. Good Luck!
Josh