Not sure what to make of this one TBH. Too many factors for me to say definitely its MC, but I'd definitely point the finger in your direction

This is whats happened to date-
I installed Vista32 as an upgrade from XP. Nothing special here, unless the upgrade process has wrecked things somewhere.
Installed a fresh copy of MC 12.0.251 on this install, along with CCP & QT/ Real alternative.
Cut & pasted the whole library/ thumbs folder back from the old Windows backup.
Launch MC- Everything runs fine.
Close MC & attempt to re-open it, and it fails to load the library. I've checked & there are absolutely no modifications made to any of the library files; They're byte identical.
Reboot the computer & MC will again open and run perfectly for the single time.
A brand-new library/ deleting the thumbs makes absolutely no difference.
Setting compatibility to XP SP2 fixes the problems, but obviously I'd rather not.
-Leezer-