Have you test the solution provide in the second post of [this](https://groups.google.com/forum/#!topic/openedx-ops/d-rmACND180) link?.
You can try going to the edx SAML configuration, find the "other config str" section, and add
```
"SECURITY_CONFIG": { "requestedAuthnContext": false }
```
Also provide the SAML response(you can use a SAMl Tracer to capture that) in case the error persist.
Thanks.