It sounds like either the forum needs to have a hidden "j_uri" field that tells the login form where to go after the login, or there must be a seesion attribute of some kind that does the same. This discussion seems to be relevant.
Be aware that j_uri is a Resin-specifc extension, and not part of the servlet specification (i.e., it won't work on other servlet containers). The Resin documentation should describe how this works in detail.
Ping & DNS - updated with new look and Ping home screen widget