Error When adding a Configuration with Partner User

Summary

Error - 500: Internal Server Error;{"type":"HTTP://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html","title":"Error: Part entered for attribute rule is not valid."}

Content

We launch CPQ from our existing R12 Application, do config selections in CPQ and SAVE the transaction. At Save action we are facing below issue,

Error - 500: Internal Server Error;{"type":"HTTP://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html","title":"Error: Part entered for attribute rule is not valid."}

Flow works fine for all Internal Users and None of the Partner Users are able to Save the configuration and facing this issue.