Would this have anything to do with the ad-blocking feature in Opera? If Opera disables java as part of its ad-blocking process, some pages or parts of pages that legitimately use java won't work.
That's what I wondered - but turning pop-up blocking off didn't help.
To my surprise though my online bank works fine with ad blocking on, despite popping windows up all over the place. Most of the time it seems to do a good job of only blocking the real ads, just a couple of niggles...