Request was from an untrusted provider

I received this error message "unable to complete request at this time. (Request was from an untrusted provider)" when I my SP app direct me to NIDP login.When I refresh , then it will only open the login page correctly. 
After I successfully login to IdP  and click the SP Web App, it redirect me to the login page of the SP App again. Please help.

  • This could mean two things :

    - the certificate of the SP is not trusted by the IDP. If using a externally signed certificate verify the CA certificate is in the "Trust" store, If using self-signed certificates add the self-signed certicate to the IDP and also put it in the "Trust" trusted store.

    - The SP sends a different Assertion Consumer Service (ACS) url then is configured on the SP in the IDP, use a browser plugin like SAML message decoder to verify the ACS url