I found this post while searching for entries with the same problem/topic "The message is not an HTTP POST" when configuring SSO and Azure. I have enabled the SAML trace as suggested above and everything appears to be fine until right after I see the following entries in the log:
4/16/2020 11:22:38 AM: Receiving response over HTTP POST
4/16/2020 11:22:38 AM: HTTP request:
GET /SSO/SAML2/AssertionConsumerService.aspx HTTP/1.1
Exception: ComponentSpace.SAML2.Exceptions.SAMLBindingException: The message is not an HTTP POST. .....
Any suggestions as to what next step I should consider?