I rebooted my machine and tried Get Messages again and it's doing almost the same thing.

It started downloading messages as indicated on the lower left of the window, and while it was doing that, it presented a dialog with "The account ... is being processed. Please wait until processing is complete to get messages." Then it finished, saying "259 messages downloaded" but they are nowhere to be found.

Gmail frequently stops allowing downloads if you use POP and have not been accessing the webmail account to delete a load of old messages that are stuck in the 'All Mail' folder. Anything you delete when you use Pop cannot put the gmail email into the gmail Trash server folder, so gmail just removes the 'Inbox' label and archives it in 'All Mail'.

This is a known thing that gmail will do to Pop users if they do not clear old mail which they think they have deleted. So I would suggest to do this now if you have not already done it.

There are very specific server settings and you cannot mix up a Pop account to use an imap server setting. Oauth2 is required for Pop and Imap and it requires a certain set up.

Please post the following information because I'm not entirely sure you set up the account correctly. In Thunderbird Help > Troubleshooting Information Scroll down to 'Mail and News Accounts' section Post an image which shows all the data in that section OR copy and paste all the data into this forum question.

Also - 115.18.0 ESR - is there a reason why you are using an old version of Thunderbird ? What OS are using?

Thanks Toad-Hall. It is now working again.

In the Gmail web interface, "All Mail", I had around 100 current emails, but had 8,000 that were November 2021 or older. I deleted all those old emails.

Concurrently, I archived about 15,000 old emails out of my Thunderbird inbox and compacted.

This whole thing must have precipitated from my being out of the country and Google changing something, security-wise.

A few weeks ago, Thunderbird was downloading to my desktop machine properly on all gmail accounts. I've got it set to delete downloaded emails from the email server. I've got K-9 mail on my Android device, and that was working too (configured to leave the emails on the server so I have a complete set on my desktop). All that was working and had been working flawlessly for years. Then I went out of the country and some (not all) accounts started giving authentication errors when I tried to view using K-9 mail. I hadn't changed anything. Then when I got back to my desktop machine, Thunderbird was unable to authenticate. All along, I was able authenticate on all accounts using the gmail web interface. And all throughout, there were two email accounts that continued to work on the desktop and mobile email client. All the accounts were configured the same and had been working fine, so Google just "decided" to screw up a few of them.

I'm using Windows 7, so having to use ESR. I know, one of these days I'll get a new desktop machine.