Is there a reason for using this old version of Thunderbird, the latest release is 102.3.0 ?
Unless you an old OS that doesn't support this it would be better to update. The reason I say this is as other programs that interact with it (like your AV) may no longer be compatible.
What version number and build of Avast One are you using ?
Recently there have been some email issues with Avast, but that was somewhat different, unable to connect. There has recently be an Avast Update 22.9 which resolved that in the Avast Antivirus branch of the AV. I'm not sure if that has yet been released in Avast One, as I don't use Avast One.