It's possible the problem is with the certificate of your email server. But they probably would fix that very quickly because it would affect all their customers, so for that possibility I'd check their support site and/or try again in a little while.

However, if you use security software that filters your internet connection, it's possible that your security software needs to be updated -- or that Thunderbird has stored an old certificate for the software that needs to be replaced with a newer one. Do you use Avast, BitDefender, ESET, or Kaspersky software?

I should mention that some higher security mail accounts might require you to import a personal certificate into Thunderbird so not only do you know who they are, but they know who you are. Does that sound like something you ever would have installed?