Is there nobody that can help? Maybe I should download Thunderbird from scratch - will all my e-mails store be there?

The following link might be helpful: Google sign in for Thunderbird https://support.mozilla.org/en-US/questions/1487655

Thanks, but unfortunately there is not anything there that seems to help.

The actual heading (on blue bar) of pop-up box is ‘Enter credentials for martinrosen1@gmail.com on pop.gmail.com’

The URL below is extremely long and although I could copy it, I cannot post it anywhere.

I have not changed any of my settings.

This seems strange. On previous occasions when I have asked a question on the Community Forums somebody has come up with an answer in a short time. No such luck this time ☹️ Can anybody suggest another place to go where I may get an answer?

... unfortunately there is not anything there that seems to help.

Did you turn on cookies in Thunderbird as suggested in the article?

I normally avoid cookies when asked whether I agree to them on sites. I don’t specifically recall switching them off on Thunderbird, certainly not recently when the problem started to occur. However, I will go there and make sure they are on.

Okay, I changed the cookie setting from ‘never’ to (Accept third-party cookies) ‘from visited’. This time the pop-up asked me to verify it is me, by my e-mail address and password. It accepted this and is now working 🤙. I will leave the setting and try to remember what I did, in case I want to change it back to ‘never’..