3666748 - Service cannot be reached (404 Not Found) when redirected to Assertion Consumer Service URL (/sap/saml2/sp/acs/<client>)

• During SAML2 flow, after authenticating with IDP, redirection to the Service Provider's (SP) Assertion Consumer Service (ACS) URL (https://<hostname>:<port>/sap/saml2/sp/acs/<client>) ends with error Service cannot be reached - 404 Not Found:

Service cannot be reached

What has happened?
URL call was terminated because the corresponding service is not available.

Note
The termination occurred in system with error code 404 and for the reason Not found.

What can I do?
Please select a valid URL.

HTTP 404 - Not found
Your SAP Internet Communication Framework Team

• In a SAML trace via Security Diagnostic Tool, there's an Outgoing AuthnRequest, but it's not followed by an Incoming HTTP request to /sap/saml2/sp/acs/<client> and there's no SAML Response.
  
  
• In an HTTP trace (DevTools > Network), there's a POST Request to https://<hostname>:<port>/sap/saml2/sp/acs/<client> with a header status "HTTP/1.1 404 Not Found". In the POST Data, a SAMLResponse can be found.

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• SAP NetWeaver
• ABAP platform

404, Not Found, ACS, ACS URL, Assertion Consumer Service, /sap/saml2/sp/acs/, Service cannot be reached, URL call was terminated because the corresponding service is not available, Please select a valid URL, HTTP 404 - Not found, external alias, No RelayState, SAML2 service not accessible , KBA , BC-SEC-LGN-SML , SAML 2.0 for ABAP , Problem