Please follow below:
Step1 - Try to login to your backend B6 system using normal GUI. This confirms your system and you have connections and access to that backend system.
Step 2 - Check the RFC Connections, GRC-AC Connector settings, Make sure the logical system and connectors and connector groups are mentioned correctly.
Step 3 - If both step 1 and step 2 are confirmed and verified, you can try again but make sure you dont have more sessions logged in backend system, you might try to log off backend system.
I believe Step-2 will identify the problem area.