You probably need to get rid of the antivirus program that is replacing it with it's own self signed one. I do not have those sorts of issues using Windows defender because it has no need to scan email to undertake protection.

Hi Matt, Many thanks for your response. I tried disabling my AV when the problem first occurred. I also disable my VPN when troubleshooting and when sending messages. The problem also manifests on my Linux machines and on the Linux laptop of a family member in a different location. I find that I can fix it by going into 'Manage Certificates' and adding an exception for a my ISP's Certificate that is already there and then confirming. I need to do that every time that Thunderbird updates. A tad irritating, but I keep smiling. Best regards, Peter.

@piw1, have you verified the cert chain?

https://www.google.com/search?q=certificate+chain+thunderbird+test+or+check&oq=certificate+chain+thunderbird+test+or+check&gs_lcrp=EgZjaHJvbWUyBggAEEUYOdIBCTExNzM3ajBqN6gCALACAA&sourceid=chrome&ie=UTF-8