2019 here, Cubase 10 Pro.
Just got my crossfade greyed out while working on the project I have to return today.
First, few days ago crossfade stopped to work while there was no overlap, today it stopped working at all.
Tried changing all my preferences to default, did not help.
EDIT: SOLVED (a bit):
Reinstalling Cubase did not help. Some preferences sit beyond the installation itself.
HOWEVER - I discovered the Safe Mode function (you have to click CTRL/CMD + SHIFT + ALT) when you launch Cubase (before the splash screen with loading appears). I disabled preferences (it resets Cubase options - even though I tried it manually in “Preferences”). It helped, crossfade works for now.