@cbareis
Strange. Chrome and Edge work as expected. Firefox (and Opera) are blocked by Bitdefender. Certifcate for exchange.opera.com is certainly neither expired nor revoked; IMO Bitdefender is at fault here.

I would contact both Opera and esp Bitdefender support, Opera issue can be reported from O menu - Help - Report an issue..., most likely Bitdefender has some feedback page either.

Nevermind, just restarted the computer and now I'm having the same issue all over again, I don't know what's going on

That's a website though, an IP address may not be the same.

As usual, as soon as I ask the question, I find an answer:
https://groups.google.com/a/chromium.org/forum/#!topic/security-dev/IGT2fLJrAeo

Now I just need a solution which doesn't involve re-building all the certificates, because some of those I use cannot be rebuilt.