The error processing URL provides the path to which a user agent is redirected or forwarded when a SAML processing error occurs. The Error Processing URL attribute is configured using the OpenSSO console. Out-of-the-box, saml2error.jsp is hosted within the OpenSSO WAR. It (or any customized page) can also be hosted with the external customer application.
If the page is hosted within opensso.war, a forward is used to send the user agent to the URL. In this case, the value of the Error Processing URL attribute is /saml2/jsp/saml2error.jsp. (This is the default configuration.)
If the page is hosted outside of opensso.war, an HTTP-REDIRECT or HTTP-POST (depending on the configuration) is used to send the user agent to the URL. In this case, the value of the Error Processing URL attribute is a URL like the following and must be modified as documented in Configuring the Error Processing URL Attribute.