Hmm.. you look fine at the admin page. Maybe bositman knows?
Maybe you're using some vpn (via opera main servers?) and we've banned that range of IPs?
What ip address do you get with
http://www.whatsmyip.org/ ? Same ip when you're using Opera and Firefox?
[edit]
Also, I just downloaded opera and used it to log in to the forum just fine, and in fact I'm adding this line using Opera. So it has to be something on your side and not an Opera issue.
[edit2]
And now I've logged in successfully after enabling Opera's VPN feature, and indeed I get an Opera's VPN IP address (185.137.19.170), but no issues. It's still possible/likely though that Opera's VPN has many IP addresses, and possibly we've banned some of them due to excessive spamming coming from them (since spammers like to hide their IP address, they possibly used Opera's VPN for that).
So if the issue is Opera's VPN, then no real solution. Try to disable the VPN while surfing the forum.