What is your version of TB, and what kind of account, POP or IMAP, and what kind of authentication, OAuth2 or normal password?

The easiest method is IMAP with OAuth2, which is the default for adding gmail accounts with the setup wizard, and using the latest version of TB.

Thunderbird said I have the latest build when I went to Help -> About and checked.

I am using the defaults, IMAP and OAuth2.

My other Gmail accounts work on Thunderbird, this one won't. I even went into the account on Gmail and allowed "less secure" devices to access it. No soap.

Login and Pass are correct. I am completely out of ideas.

Did you remove all passwords and oauth:// tokens for the account in Tools/Options/Security/Passwords/Saved Passwords, restart TB, and then add the account? Old or obsolete password entries can cause the account setup to fail.

Less-secure apps access can be left off if the account uses OAuth2.

I removed the account entirely and tried to re-add.

I can look if there's any leftovers and report back.

OK, no good. I see nowhere to actually look since the account was entirely removed.

No good. I also tried a restart.

This is what I get...and I KNOW the login/pass is correct

Exactly the same for me. Help!

I have the same problem. I try to log in but the gmail login screen gets stuck in an infinite loop. I enter my email, and then it refreshes and then asks me to enter my email again. It never asks for password. I am not sure how to fix this problem.

I have a similar problem. "Mail server pop.googlemail.com responded: Username and password not accepted." Mac High Sierra OS.

Me, too. "Mail server pop.googlemail.com responded: Username and password not accepted."