Unable to decode incoming request

[Peter Schober]

* Ryan Suarez <ryan.suarez at sheridancollege.ca> [2019-07-16 17:33]:
> POST https://my.idp.ca/idp/profile/Shibboleth/SSO

That's your endpoint for proprietary "Shibboleth" protocol requests
under SAML1.

> <saml2p:AuthnRequest
> Destination="https://my.idp.ca/idp/profile/Shibboleth/SSO"
> ForceAuthn="false" ID="LI_qd47rn684941iui6hebvqmdqc4"
> IsPassive="false" IssueInstant="2019-07-16T15:18:29.733Z"
> ProtocolBinding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST"
> Version="2.0" xmlns:saml2p="urn:oasis:names:tc:SAML:2.0:protocol" >
> <saml2:Issuer
> xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion">https://some.sp.com/someEndPoint/ABEAAAAAAAAirCEAAAAAAuNRbAHzAzyYcMFFTC97fZ9C4TfaBxvDtg</saml2:Issuer>
> <saml2p:NameIDPolicy AllowCreate="true" /> </saml2p:AuthnRequest> 

But the authentication request is using the SAML 2.0 protocol.
I.e., the SP is configured with the wrong endpoint for your IDP.

-peter