Hi,
do u have import the Portal certificate into the AS ABAP tcode: /strustsso2.
you can export the certificate at the nwa: http:<host>:<port>:/nwa/key-storage
Choose the "TicketKeystore" and then export the "SAPLogonTicketKeypair-cert".
ABAB System:
1. Import certificate (from the portal)
2. Add to certificate list
3. Add to ACL
Try SSO again.
Additional you can add the Backend System as trusted: http://<host>:<port>/nwa/trusted-systems
br,
fabian