open epic fhir sandbox is throwing { "error": "invalid_client", "error_description": null }

Question

I'm trying to use the sandbox from https://fhir.epic.com/ for Backend Services.

I am following this tutorial : https://fhir.epic.com/Documentation?docId=oauth2&section=BackendOAuth2Guide :

I already register a new app, created a JWT (using SSL keys) tested the JWT on https://jwt.io/ : works fine! But I cannot POST the JWT to the endpoint to obtain the access token. I should send a POST request to this URL: https://fhir.epic.com/interconnect-fhir-oauth/oauth2/token.

The app is working for 8 months and suddenly stopped working and throwing below error.

{ "error": "invalid_client", "error_description": null }

Tried creating new app, created new certs but still no luck.