It is the location that the certificate is for I would guess. Perhaps you might explain exactly what sever your trying to connect to. there is just not enough information to do more than ask for more.

We most often hear of there sorts of things when folk have a hosted domain and use a shared mail server using an SSL certificate for the host. It will not match the mail server you are reaching out to but an exception is reasonable because you expect it to not match. It can also be when SSL/TLS encryption is used on servers that are not configured for it do the domain in question. Many hosting services do things on the cheap and mismatched SSL certificates are a frequent byproduct of that.

Saving inability usually means the certificate is not a 2048bit certificate as required so the exception can not be saved. It is the information under view that is relevant as well as the server you are trying to connect to.

Thank you very much for your insight. In the meantime I solved the problem by editing the cert_override.txt file as suggested as a solution for Thunderbird version 78. For some reason or another it did not work the first time I tried it.(probably an editing error on my part). A later attempt solved the problem.