I have implemented a custom auth provider as part of the single sign on flow for Salesforce communities. Most of the flow is working correctly. I am able to get the authorization token, access token and the user information. When the custom IDP redirects back to Salesforce communities, I get a 503 error (service unavailable). According to the debug log, the flow is hitting the Registration Handler class. I have commented out the code that creates a user if a user does not exist. I am simply query for a sample community user and returning it in the Registration Handler. However, I am still getting a 503 error. Any suggestions?