Most likely the URL to hit the webdispatcher is different than the one to hit the appserver directly. This new URL will not resolve to your enterprise cluster. Most likely hitting the system default cluster where you would be able to login only as the system user. The system and enterprise users use a SAP Sourcing login page since the credentials are maintained in the app database. To add webdispatcher you will most likely need to adjust the hostname/port in enterprise cluster.
Regards,
Gary