Please tell us, does this help https://www.reddit.com/r/Thunderbird/comments/kd33xf/thunderbird_cant_connect_to_gmail/gfvydsc/?utm_source=reddit&utm_medium=web2x&context=3

Hi,

In Google security settings:

"Click on the Third-party apps box, then pick Thunderbird. (Mine says, "Mozilla Thunderbird Email Has access to Gmail".)"

I don't have Mozilla Thunderbird in the list of 3'rd party apps. I guess this could be part of the problem....

I had issues with gmail server the other day. It also gave same error message. Checked online and there were loads of people in the uk suddenly unable to connect to server. https://downdetector.co.uk/status/gmail/

I tried to logon to webmail and got an error with that as well. So I waited for a couple of hours and it got up and running again after I'd needed to logon to everything again. So this could be gmail having an intermitant problem.