When running the sheduled job in FME Server, we are encountering this error from time to time. Today, FME Server 2016 cannot connect to ArcGIS Online.
ArcGIS Online Feature Service Writer: Encountered an unexpected error. The error code from the server was '403' and the message was: 'You do not have permissions to access this resource or perform this operation.'. Details: '
However, I tried running this in ArcGIS Desktop 2017.1 and it was able to connect and write the features to ArcGIS Online. I'm using the same ArcGIS Online account for both.