Hi djjnc,
We'd like to help you find the best solution path for this issue. You may need to reach out to the website developer, but we'll see if our guidance can help. We recommend starting with verifying that your date and time settings are showing accurate for your location and to disable any VPN connections if you have one. If this does not help, try to Clear your browsing history in Safari on Mac. If you still have an issue, see if this happens with another internet connection to verify there is not a security checking issue with your network and see if another user account on the Mac helps.
We would like to know the results and can help provide additional guidance if needed.
Thank you for using Apple Support Communities.