I know the cookies thing is a common problem, and here's mine! Here's the scenario:
After clearing all related cookies, both Firefox and Internet Explorer only allow me to log in when "Remember Me" is ticked, and are not allowing me to post (get the "authentication" error).
I dont know how to solve your problem the way you would wish, however, you can get rid of the post authentication issue by switching back to the People.Class.Authenticator.php instead of the People.Class.WordpressAuthenticator.php.
Sorry I can't be anymore help, trying to solve this too myself. At least this way, you can still use your forum!