Sorry that it's been so long since you got a response. It looks like you've hit everything I'd normally suggest looking at. Double check that:
* the active checkbox is checked on both the app and the service
* the app has a default role on the service that works
* the role has at least GET permissions on the service. Make sure permissions are assigned on both service/ and service/*
For troubleshooting, I'd set a default role on the service for the application. Go to the docs and get the /_proc or /_schema url. Add ?api_key=[longAPIkeyFROMappHERE] using the API key for the app to the end of that URL and paste it into your browser in private mode to see if the permissions are right and ensure that the service is responding. The idea is to narrow down if it's an issue with the user permissions or an issue with the app or service.
You might also try clearing the cache in config.
Hope that helps!
Josiah