i just bumped into this problem. well in my case it was easy to fix
i was using <login-page>/login.html</login-page>(wrong) instead of <form-login-page>/login.html</form-login-page> form some reason the parser didnt threw me an error
This error occurs when the container is unbale to pick up the login page at runtime. I would check that i have the right mapping in web.xml and the page is there in the right directory ect.
SCJP 1.4 , SCWCD 5.0
subject: WARNING: Unexpected error forwarding to login page