Adding a new gmail IMAP account will fail if the authentication is OAuth2 and your version of TB is not current, i.e. 60.9.1 or 68.3.1. Does that apply here?

I checked and have 68.3.0. I'm guessing its the latest update, Thunderbird didn't show anymore. My main gmail account set up just fine, but when I went to add the second one is when I started getting the error.

Is the second account POP or IMAP? If it's IMAP, it should add without issues, with the default settings. But if it's POP, factors such as less-secure apps, 2-step verification, recent mode require more attention.

It is IMAP. That's why I'm confused. Both accounts worked fine on my new laptop, but I can only get the one added on my desktop computer.

Both my gmail accounts were working just fine until this afternoon when one suddenly stopped receiving incoming mail. Gives me a message saying that the server doesn't recognize the password.

I checked both the version of Tbird (68.4.1) and the security/password settings. Both accounts are set to OAuth2, the passwords are correct and saved in the security settings. But one works and the other does not.

I can only get my mail online or on my phone or on another computer but not on my laptop.

I suspect this has to do with a recent update. Can I remove the most recent update?

Solution??

Success and here is the answer. Somehow I got signed out of the Google account that stopped working. I had to sign in, reauthorize Thunderbird to receive mail, verify I am the owner and it works now.

I've rebooted my computer, resigned in to Gmail via a web browser and verified my name and password. Nothing has worked.

I have two IMAP gmail accounts and Tbird 68.4.2. Tbird can login to only one of the two accounts because I have to be logged in for the download of email to succeed. But I cannot be concurrently logged in to two different Google accounts.

This is a Tbird coding problem. Everything worked fine until the latest update.

I am having this problem, too. I have several Gmail accounts set up and all were working fine. Two of them are school accounts (Capella) that use Gmail. Yesterday, I changed the password on these two accounts at the school website. I can login just fine through my desktop browser (Chrome, Windows 10). One of the accounts is working fine in my Thunderbird. One isn't. I deleted the one not working and tried setting it up again. Everything is set up correctly, yet I get same message as the OP "Unable to log in at server. Probably wrong configuration, username or password."

This same problem was solved for me by sfhowes:

Cannot set up GMail in Thunderbird sfhowes Feb 15, 2020, 3:51:39 AM

Start your computer in Windows safe mode with networking to eliminate interference from any startup security apps. Open TB and Tools/Options/Security/Passwords/Saved Passwords, delete all passwords, restart TB. In the Server Settings for each gmail account, set the authentication to OAuth2. If you have gmail POP accounts, update to 68.5 which support OAuth2 for gmail POP. Set the authentication for all smtp servers to OAuth2. Check that cookies are allowed in Options/Privacy. When you connect to an account, enter the regular account password in the OAuth browser window (not an 'app password').

http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite

If this works in Windows safe mode, either remove Norton AV or add the TB profile folder as an exception or exclusion in NAV, and disable scanning of secure (SSL) connections.

I am facing a similar problem. Connecting through TB on computer A works just fine but on computer B I getting an error when I try to set up the account.

Any further inputs?