This one only seems to affect konqueror- which I know is an unsupported browser. Logging in worked fine up until a couple of weeks ago, now once I've logged in, instead of auto redirecting me to the forum, it brings up a dialog asking if I want to resend the form data. The way I get to a forum is by clicking the "Back to the forum" link.
Odd.
This only happens when logging in, all the posting stuff with works fine without me having to manually click the link.
I haven't modified the forum since adding the Private messages functionality which was quite some time ago.