Configuration Issue reported on load

I run MM on desktop (W11). I always get an error reported which I close and generally everything loads and is ok. The error is this: Mattermost Discussion Forums
the text is
’ There is a configuration issue with the mattermost server, or someone is trying to intercept your connection… ’
then this is probably the clue ’
Certificate is different from previous one.’ and 'Error: net:: ERR_CERT_COMMON_NAME_INVALID

I suspect is is related to an MM upgrade a while back, but only now do I have time to look into it.

What is the certificate and how do I fix this?