Can't post preview or submit new thread.

I'm trying to post a question to the newbie forum. When I click 'submit' or 'preview', instead of displaying my message, I'm forced to log in again. But I can't do this because the login won't recognized my password, the same one I used to login to the forum as a whole. I was already logged into the forum when this happened, and I tried to log in with the proper password a few times to check I didn't make a typo. What's wrong? How do I fix this?