I have the same problem at home: the CROSSFADE does not work. It happened all at once. Reinstalling the software, returning preferences to default: nothing helps. Broken. Unable to activate the “X” shortcut or to find the “Crossfade” menu. W10 and latest version of Nuendo, does anyone have any idea? I really do not understand and stuck in my job. What’s wrong? 
EDIT: I have completely cleared the preferences as advised. It worked, I got the Crossfade function back. Woo what a problem say so. 