This can be fixed by:
1. Setting ProxyPreserveHost on on Proxy end. As per:
http://wiki.scn.sap.com/wiki/display/BSP/Using+Proxies
Rule 1:TheHost:headermustbe preserved by proxy. The proxy must not in any way change theHost:header.
The Web Dispatcher will always preserve the Host: header, and works correctly on this aspect. The Apache proxy can not in the version 1.x preserve the Host: header, and can not be used with the WebAS together. Only from Apache version 2+ is it possible to configure an Apache so that the Host: header is forwarded unchanged. For this, set the configuration option ProxyPreserveHost.
2. Configure SAML2 by accessing the reverse proxy URL with host/port that would be used by IDP/ end users, as per wiki link shared in first response above.
