A bit more detail:
When I first installed Avast One Essential (around beginning of April) as a replacement for the old Free Antivirus, I did not initially have the problem with Gmail. It worked for about 10 days. Then the failures started. In troubleshooting, I did a complete uninstall of Avast One and deleted all the Avast root certificates from Thunderbird, including the old Avast Free ones, and the new Avast One certificate. After reinstalling Avast One, a single certificate showed up in Thunderbird, but the problem remained.
I have now tried running avastclear.exe to do a total uninstall. After the uninstall, I went back into Thunderbird and deleted the certificate, which does not appear to be deleted by avastclear. I then reinstalled again. The problem is still there.
So, back to disabling the Email Guardian. I hope you can find a fix.