I have removed AVG antivirus because it was redundant.

So what AV software are you using now?

Windows Defender

Try to start Windows 10 in safe mode with networking enabled.

Does the problem go away?

Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.

I tried it in safe mode and didn't see the problem.

You should know the drill by now: https://support.mozilla.org/en-US/questions/1516342#answer-1743699

Christ1,

It looks like you were right, it was the anti-virus software. Since this is the first Windows computer I've built since Windows 7, I didn't realize until recently that Windows installs its own anti-virus software, hence the use of AVG, which was never the problem. Once this came to my attention, adding the needed exception to Defender solved the problem.

Thanks for your help with this problem.

And now that I've declared the problem to be solved, it's back.

Anyone have any other ideas about this problem?

As a workaround, you may want to try using an app password for the Yahoo account. https://help.yahoo.com/kb/password-sln15241.html

Are you having any other email accounts using the Yahoo servers, e.g. like AT&T, Verizon, AOL, etc. set up in Thunderbird?

Yes, I asked Chatgpt about this and the app password is what it recommended. So far, it seems to be working. The bad news is that the change corrupted my folder structure and I ended up having to completely delete and add back the account to get the folders back. Thanks for the advice.