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In the server.log, the error "Unable to lookup sp connection metadata for entityid" is seen. This is usually an indication that there is a mismatch between the Partner Entity ID (Connection ID) configured in the IDP-side PingFederate SP Connection and the actual entity ID of the partner, and therefore PF cannot determine which SP Connection to use when a SAML AuthnRequest comes in from the SP in the SP-initiated SSO use case.
Since Entity ID is case sensitive, if there is a mismatch between the value entered for the Partner's Entity ID (Connection ID) field in the PingFederate Administrative Console and what the partner is sending in the SAML protocol message, then the SSO attempt will fail with the "Unable to lookup sp (or idp) connection" error message.
The solution is to verify the Partner's Entity ID (Connection ID) setting matches exactly what is sent by the partner in the SAML messages.