Ok, I'm not 100% sure, but this sounds like a security type setting in your browser. Have you been able to reply to any ad on the site since this message started to appear? Can you post a link to the ad? In the meantime, try sending the seller a PM to make contact with them.
What I would suggest is that you clear your internet cookies that are temporarily stored on your computer, and then visit the site again. You will need to login again with your username and PW... Then try to reply again and let me know how you make out.