I have been getting this same error message every single time I've tried to submit a post (using the standard Post Reply or Quote button) over the past week. If I click the back button, then refresh, then submit again, it goes through. But each time I hit Submit the first time, I get this message.

I don't know if this is related, but there was a recent update to Firefox at about the same time this problem started happening. I am currently using Firefox 3.6.