You have configured SAML Authentication in AEM and after logging in to the IDP you get a 400 response from AEM during the SAML POST to /saml_login or /content/saml_login (or other URL configured for the iDP to post back to).
The problem only happens when authenticating via the dispatcher enabled URL.
In the dispatcher.log, you find this text:
Filter rejects: POST
/0100 { /method "POST" /url "*/saml_login" }