I propose to backup then uninstall Tbird and re install. However not sure if this will reset everything.

You'd be wasting your time. Profile files are stored separately from the Thunderbird program files. The program files are static and do not change (except when you upgrade the Thunderbird application version). Because we store profile and program files separately, you can uninstall Thunderbird without losing your messages and settings, and if something goes wrong with a Thunderbird update your information will still be available. It also means that you don't have to reinstall Thunderbird in order to delete your information or troubleshoot a problem.

What version of Thunderbird are you running?

What is your anti-virus/security software?

Thanks christ1. The details you asked for are: Thunderbird 91.4.1 Running under Windows 7 Home premium Servive Pack 1. Bullguard was uninstalled about when email downloading ceased but error message started appearing a week or so before. AVG free is current antivirus which was installed before bullguard used.

This could indicate that something on your system or network is intercepting your connection and injecting certificates in a way that is not trusted by Thunderbird. The most common causes are security software scanning encrypted connections, or malware listening in and replacing legitimate server certificates with their own. To disable scanning of secure connections in AVG see https://support.mozilla.org/en-US/kb/error-codes-secure-websites#w_avastavg

Wrt AVG also see https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVG

Thanks again Christ1. I looked at the links in your reply and followed a lot of the links in these. It seems that the issue is complicated and beyond my knowledge. I disabled scanning of secure (HTTPS) connections in AVG, loaded Tbird and no change, so I put back the AVG check. Previous to this query I had run Thunderbird in Troubleshoot mode without effect. It seems that running windows in safe mode is another possibility. I am not sure of this. Going back to my original query: rather than uninstall thunderbird could I backup then delete the two profile files? Would Thunderbird reset the basic information in them? Where would it get this information from? I could then reset up the accesses to my email accounts hopefully with new certificates installed. I note that there are two profile files in windows7 under \AppData\Roaming\Thunderbird\Profiles\ and \AppData\Local\Thunderbird\Profiles\. The Roaming one, (as it is big), seems to hold the emails that I would like to retain. Does the local one hold, among other things, the certificates? Sorry so many questions. I am trying to get my head around it.

Going back to my original query: rather than uninstall thunderbird could I backup then delete the two profile files?

You should backup the Thunderbird profile on a regular basis regardless. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

There is no need to delete an existing profile. Just create a new one using profile manager, and start Thunderbird with the new profile. https://support.mozilla.org/kb/using-multiple-profiles

You can then recreate your existing email account in the new profile and see if it works any better.

I note that there are two profile files in windows7 under \AppData\Roaming\Thunderbird\Profiles\ and \AppData\Local\Thunderbird\Profiles\.

Neither of these are the actual profiles. Profile folders are located underneath AppData\Roaming\Thunderbird\Profiles though. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

There are no actual profile folders underneath AppData\Local\Thunderbird\Profiles, even though the folder names are the same as the actual profile folders. Primarily cache data is located there. In a word, data which doesn't need to be backed up.

Does the local one hold, among other things, the certificates?

No.

It seems that running windows in safe mode is another possibility. I am not sure of this.

Running Windows in safe mode with Networking enabled will disable anti-virus software, among other things. So it's not a solution but a troubleshooting step to determine whether anti-virus software is the culprit.

Non-overridabe TLS error occurred. Handshake error or probably the TLS version or certificate used by server imap.mail.yahoo.com is incompatible.

I've also seen this error now in connection with a Gmail account. It was temporary, and disappeared after restarting Thunderbird. I have no idea what this is about, but my best guess would be a temporary glitch on the server side.