Avast blocking thunderbird mail

I cannot send or receive emails through thunderbird since avast is now suddenly blocking all emails
I get the following message when trying to send
Sending of the message failed.
Peer’s certificate has an invalid signature.
The configuration related to …(my server) must be corrected

I tried updating avast, then exporting a certificate from avast (but thunderbird won’t accept it, saying:
This personal certificate can’t be installed because you do not own the corresponding private key which was created when the certificate was requested.

I’ve looked in the forum and seen endless similar posts going way back, how is it avast is still creating these issues, can anyone suggest a way to fix my issue?

This may help
https://support.avast.com/en-ww/article/91/