Thunderbird 91 - Add Security Exception

Thunderbird has stopped pulling my email from the two accounts I have setup, each on a different domain/mail server and hosted by two different web hosts. I get the "Add Security Exception" for both email accounts. I have made no changes to either account in recent months, and no changes to Thunderbird, other than to installed the recommended updates. Both accounts were affected at the same time. No antivirus apps installed... only the native Windows Defender. Why is this happening, and how do I fix it? Of course, I can always just go for the security exception. But, I don't think that's the way it should work. I don't believe the SSL certificates would have expired on both domains at the same time. They are unrelated. Any help appreciated.