It's an empty gray rectangle in the middle of the screen instead of it. The film strip and all the panels are visible.
Is one of those filmstrip images highlighted?
The situation is following. As soon as one of the filmstrip images gets highlighted, it gets gray. A big single open image remains OK until any editing applies. As soon as I want to work on it it gets gray as well. Then they are both gray - a filmstrip and a big single image. But when the cursor moves to another image these get visible again and the next one has the same problem then.