Unable to access any websites on Mac

[strumpusplunket]

My wife has a 2013 iMac, which she uses in our home office for her work computer. It's running MacOS 10.15.5.

Over the past week we've had an issue where anytime she tries to access a website, we get a message that the site's certficate has been revoked, and the site won't load. This happens in both Safari and Opera.

I am assuming something has gone wrong at the OS level, but I don't know my way around MacOS at all, and have no idea where to start looking for a solution.

Does anyone have any suggestions for anything I might try before I just nuke it and reinstall the whole OS again?

 

[strumpusplunket]

is the time and date correct?

Yeah, that was the first thing I checked.

On fruther investigation, this looks like it might be something to do with a dud cached certificate or two somewhere in the chain of certification. Some sites are now working, so I am surmising that one or more of them has now expired from the cache and been reloaded. I'm now looking into how to purge the cache of certificates, but I don't know my way around MacOS at all, so it's a slog tbh.