Setup SAML with Salesforce IDP gives error "Error: Invalid HTTP method"

SYMPTOM

New SSO setup using Salesforce IDP. On attempting to log in you get an error like below

"Error: Invalid HTTP method"

URL of the above page looks like https://.....saml/SamlError?idpError=1605
 

CAUSE

SOLUTION

Sometimes the IDP has two single sign-on URL try changing the single sign-on URL on Anypoint to on of the below URL. If one does not work try with the other.

<md:SingleSignOnService Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST" Location="https://abc.com/idp/endpoint/HttpPost"/>
 
<md:SingleSignOnService Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect" Location="https://abc.com/idp/endpoint/HttpRedirect"/>