The screenshot is now attached. Thanks

is there a way of changing the language of Thunderbird from Russian to English without reinstalling and reconfiguring all email?

Download the English installer and run it. https://www.thunderbird.net/thunderbird/all/

Then post your screenshot again.

What is your anti-virus software?

Thanks! Please see the image in English attached.

I use Avast antivirus.

Inspect the certificate - see attached screenshot for instructions. Who is the issuer of the certificate?

Please post another screenshot of the Certificate Viewer window with the cert issuer information visible.

If the issuer is Avast, see https://support.mozilla.org/en-US/kb/error-codes-secure-websites

Thanks. I have made the changes that you have suggested and changed the settings in Avast. Instead of 4-5 consecutive messages like this, I now get one or two, which I suppose is an improvement. But it has not been possible to eradicate them altogether...

This is an issue with Avast. Personally I would just stop the product from scanning mail.

Thanks, but I could easily get a virus in that way, no?

You did not post the cert issuer information.

I could easily get a virus in that way, no?

No. I'd argue you're way more secure without letting Avast intercept your secure TLS connection. And it would still protect you if you attempt to run or save a malicious attachment to disk. Not even talking about that you'd prevent Avast from corrupting your mail files.

I still don't know why people think installing a 3rd-party anti-virus software is a good idea anyway, while the built-in Windows Defender is all you need.

nkazimirov said

Thanks, but I could easily get a virus in that way, no?

No, like Christ1, I do not support email scanning.

Thank you both for your help!!