Debugging Error: com.appiancorp.connectedsystems.http.execution.AppianHttpRequestExecutor

Hi,

I have an Appian WebAPI that performs a DB write using standard Appian smart service. During regression testing it was observed that sometimes the API fails with a http status 500 for a request that otherwise works perfectly.

On further investigation I noticed the below error in tomcatStd.log

[Appian AppianServerThreadPoolProvider 4577208] ERROR com.appiancorp.connectedsystems.http.execution.AppianHttpRequestExecutor - ConnectorRuntimeException [title=You are not permitted to perform the requested operation, The external system understood the request but refused to authorize it]

The API is authenticated by oAuth 2.0 clientId and the service account does have access to all the necessary objects required for execution. Does anyone know how to drill down this error to find the component that is denying access?

Thanks!