I don't know if it will be any help, but...

I used to get the exact same error when I first signed up for this board. I was trying to post from home on a Win2k box running IE 5.5. Through suggestion, I managed to single out the software firewall (ZAPro). I fiddled with options that I could set specific to the website, and was finally able to post once I turned off a privacy option called "Remove private header information." And according to the firewall, that is cookie related.