Just tried it on a few forums again just now, and no problems this time.
A few weeks ago though, I got a message from a visitor who couldn't register because of the captcha-error.
I tried it myself and experienced the same problem. So tried a few more UBB powered forums and noticed the same behaviour.
I don't remember which forums though. I didn't pay attention to the URLs then, just to the subscription itself.
Don't know if FF has anything to do with it, but by now I have upgraded to FF 3.0.10. At the time of the problem, I was on 3.0.8.
I am using Eset and Zone Alarm for security, but those haven't changed since my initial post here.
As said, I cannot reproduce the problem anymore now, but I will keep an eye on it and try to figure out some more info if I run into it again.
