I figured I'd blow the dust off of this thread and see if anyone ever found a fix for the problem.
My issue appears to be the same as the OP.
In FTM 2012 when categorizing media files, at the end of processing a small batch (less than 10 at a time) I'm left with the collection screen showing a blank screen with a large red X in what should be the collection screen's center thumbnail view. The thumbnails seem to actually be there although they can't be seen. If I hover my mouse over a blank spot on the screen I see the little ribbon that gives the name of one of the media files that would ordinarily be displayed.
I've tried refreshing the screen, moving to another tab and back, and opening another tree before going back to my original one but the only thing that seems to work is to exit FTM completely and go back in.
Anyone stumbled across a fix or a workaround short of exting and restarting the application?