If you are using ESET/NOD32 security/AV then probability is high that ESET is causing your problems. According to ESET forums, you should temporarily disable SSL on ESET until they solve the problem per https://support.eset.com/en/kb3126-disable-ssl-filtering-in-eset-windows-products.

You might also inform ESET that they have broken your system. The problem is being reported by hundreds of users https://forum.eset.com/topic/26517-problems-with-thunderbird-and-imaps/ https://forum.eset.com/discover/

Thanks for response, unfortunately I do not understand the term 'ESET' Regards

Thanks for response, unfortunately I do not understand the term 'ESET' Regards

Then perhaps google it. or click the links.

What anti virus do you use then? given you have no idea what ESET is I must assume you do not use it.

I use Bulldog antivirus software,

have you tried disabling bulldogs email scanner... often getting rid of the anti virus fixes all sorts of issues with retrieving mail as it acts like a dam if it is not working correctly and blocks the flow of information.

Unfortunatelly this did not solve the issue, many thanks for your efforts