A long time ago I had something which sounds like a similar problem.
The trouble was not with AVAST or Thunderbird it was due to some update my email service provider (my ISP) had done. When I reported it to them, suggesting it was a certification issue, the problem was resolved a few days later without any explanation.
I suggest, if your ISP has a forum, you check there for anyone having similar recent trouble with Thunderbird or use other means of communication for problem reporting. Describe the issue, particularly the possibility of the involvement of the mail server certificate and AVAST's Mail Shield then maybe you'll get some joy.
You've reported it here so one of the likely party's involved is now aware there may be a certification recognition matter. So reporting it to your email service provider is the obvious next step.
But don't expect anyone to enlighten you as to what the problem was and who was to blame. Just hope that one or the other resolves it quickly.