I googled “checking itunes music library” and your link was the first to come up as “scariest thing to see ever.”
It’s actually REALLY easy to fix when you see this and requires no itunes rebuild. All you do is trash the iTunes Library Genius.itdb and reopen itunes.
That’s it. Easy breezy lemon squeezy. Try it next time it happens, or better yet. Don’t use Genius because it is notorious for causing problems.
Hope that helps!
I haven’t tried it yet, but if you’re having this problem this may work for you.
It’s amazing to me that the Genius database, which can easily be rebuilt, is responsible for the scariest dialog box I’ve seen on a computer in the past few years. I wonder how many people hate iTunes and Apple because of it.