This folder is being processed. Please wait until processing is complete to get messages. CAUSED by multple (2-3+) Thunderbird windows

This error won't go away. When I start Thunderbird, I get three windows opened at the same time. If I close one, the others close. Each window has the same message: "This folder is being processed. Please wait until processing is complete to get messages." Otherwise, thunderbird seems to work ok.
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