It's the biggest folder I've got by a long shot. (Over 5k messages, large but not unreasonable.) I can't copy or move anything into it. When I select it, I see nothing in the preview listing, not even a message saying the folder is empty.
When I look at the .evolution/mail folder, I see all the message still there in the right folder. When I right-click the folder and view properties, I see the correct message count.
Any ideas?