I still cannot access the site using Safari on my iMac. I can using Firefox. And I can using Safari on my MacBook Pro.
Both devices are running Catalina and are up to date.
On the iMac I get the same message as golunvolo.
I have cleared caches, cookies and history, shut down, restarted, several times.
I made an exception for the site certificate when the https problem arose earlier, but I do not know how to remove the exception. None of the certificates in my Keychain seems relevant.
Assistance would be appreciated.