I been having this problem for while some times Gmail threw Thunderbird, if it wont receive or send gmail threw Thunderbird when it happens , I can either shut off avast shield and it works or I can force definition update and that might fix it. been happen since 20.5.2415.
No error no warning it just blocks it, I had to turn off avast shield this morning again to just get my gmail, after I turned it back on it was working again.
8U4N8
20200729_1655_8U4N8_2273704719.zip
was file that got uploaded