SP-initiated SSO is failing with error 400 - Reason: UNSUPPORTED_AUTHN_REQUEST_BINDING.
Siteminder 12.52 release onward supports SAML 2.0 HTTP-POST "Authentication Request Binding" in addition to HTTP-Redirect Binding.
With older Siteminder release (IdP), that does not support SAML 2.0 HTTP-POST Authentication Binding, HTTP-Redirect Binding is used by default.
Hence, if Service Provider sent AuthnRequest via HTTP-POST binding, Federation login failed at Siteminder (IdP) with this error 400.
If you are getting the same error with Siteminder release that support SAML 2.0 HTTP-POST Authentication Binding, it's likely that IdP has not configured to allow HTTP-POST Authentication Binding.
If you agree with SP sending AuthnRequest using HTTP-POST method, please allow it in your IDP configuration.