Hi there and welcome
I need to see the certificate details myself, can you give me the website address so I can check the certificate please?
yes, no issue at all with the certificate, usually an issue with the Mac keychain on the clients computers
Have them do this please
In keychain access, go to View -> Show Expired Certificates. Then in your login keychain highlight the Certificate and delete it.
Then restart and it should stop the error
Let me know how you go please
No nesesarily, depends on what is actually at fault with the certificate.
It is not due to a firewall, they do not deal with certificates, that's the browser. It also does not work on yours correct?
I tested it on high and low settings and never could get any errors to come up
In the end though it is not something that should be happening with a certificate and it is the certificate at issue (as again you also get the error)
But try on your computer going to Safari ▹ Preferences ▹ Security
If you reduce it there, does it stop the security
Also, do you have any anti virus on the mac?