Make sure the Username field has your full email address, not just your name.

It does not help. Of course, I know how to enter this field (which is named "e-mail address anyhow). Thunderbird then correctly gives the IMAP settings and sends a message to Google to request an authorization. The Google authorization popup windows opens, but when I click on Authorize, Thunderbird says the connexion to the server is impossible... Popup windows has url https://accounts.google.com/signin/oauth/legacy/consent?authuser=0&part=[756 characters] Tried to do it while already connected to my google account, with no success.

Make sure cookies are allowed in Thunderbird. Do not use a VPN while attempting to authenticate to your Google account. Make sure there is no application running on your computer which is blocking local TCP port 80, e.g. like a local web server.

Thanks for your help, this time it worked. I would not have been able to find out without your help

I had xampp active on my machine and using port 80 I did sudo lsof -i -P -n | grep LISTEN | grep 80 to find out I had 8 lines of this kind as a result /opt/lamp 62826 daemon 4u IPv6 19007 0t0 TCP *:80 (LISTEN) I did sudo /opt/lampp/lampp stop to stop the xampp programm (and had no longer any daemon listening to port 80

Then I tried again to add my gmail account and it worked seamlessly.