I think that I've found out what the problem is. It seems to be an issue with the (Online Certificate Status Protocol) stapling due to this website's SSL/TLS certificate, which might be outdated or something.
I've found a temporary solution for this here:
https://support.mozilla.org/en-US/questions/1028755
And as I've mentioned before, this is the only website which seems to have this error, at least for me anyway. Today, I wasn't even able to access this site at all until I implemented the above solution. However, it seems best to leave this feature on rather than off permanently, so hopefully this issue with the site's SSL/TLS certificate will be fixed soon.
Edit: Here's the error message that displays when trying to access this site normally without the above fix:
