Thank you, that is actually a professional approach, i must do that. In the mean time I found an opera://flags#allow-insecure-localhost flag, but I'm not sure it works for sites other than your localhost.

At the moment I successfully tested with the --ignore-certificate-error argument for path\launcher.exe of Opera.

Even with the wireshark approach, my fear is that there may be more than 1 CA contacted by the browser before it loads the webpage, which would mean more holes in the firewall as number of pages with different CA gather. (or just my thought on this).

Anyway thanks for your help.
All the best!

Hi, i'm trying to setup a computer that loads using Opera 42 browser a webpage at startup. It has all outbound traffic blocked in Firewall and I manually add exceptions. The webpage that tries to load give me a Server Certificate invalid (although it has a valid certificate). My guess is that the Opera browser does a check and because the firewall blocks the request, i get that message.
Can anyone please help me out with information pointing to the mechanism Opera uses to check a website's certificate?
I found something about sitecheck2.opera.com should I whitelist the IP behind that?
Or if I can turn off the Certificate check somehow... or add an exception. I find the new Opera browser incredibly .... how to put it properly... "lame" i guess...?... Could not find anything in Settings or opera://flags ...
anyway, thanks everybody. Happy New Year