Thanks. But the cover has not seized up. It moves, back and forth, but rather than unscrewing, it comes up against some sort of barrier that prevents it from turning further, something like the devices used on medicine bottles to keep kids from opening them. The oven is fairly new (3 or 4 years) so I suspect there is some sort of mechanical gizmo that keeps it from turning, not just a lot of old grease. Well, try again.
I finally got it unscrewed by using a bit of WD-40 (not sure it was needed) and using a screwdriver against one of the nubs on the cover, tapping it gently with a rubber mallet. Thankfully it did not break, and we are back in business again. But you were right, it had seized.Regards