This is (e.g.) my profile in Opera 12.16:
Other gravatars are also affected, but not in SE Chat.
When I copy the image URL, I get
Chrome retrieves the image, no problem, but Opera says:
Edit In the interest of meta overflow a screenie taken in Chat, clearly demonstrating that chat displays the avatars fine, but not when oneboxing...
I'm not fully aware of what online certificate validation (OCSP) is, but it sounds like something worth investigating?