I've had that problem too when i get to be a little too eager to post. I think it's a failsafe feature to prevent spamming. But after giving it a few seconds (about 10 to 15 or so) it works fine for me without having to close the browser.
Try slowing down when you post. If that doesn't help, then there maybe something with the forums. For what it's worth, it seems there have been alot of glitches with the forums lately.