Which version of FME Server is this?

FME Server 2019.1.2

Thanks

Any proxy settings?

Also, what happens if you enable debug logging in FME Desktop before republishing to FME Server? The log should hopefully contain more information.

I believe 2019.2 has retry logic in the AGOL reader/writer to help with this issue. If you can capture a detailed log like david suggests and open support call with Esri giving exact timings they can comment on 50X errors more.

OK tell Esri your org ID, the service and layer being written to and see if they know of any issue.

It's a known error produced by ArcGIS Online that esri knows about. Here's a list of things I would check...

1) Check your readers/writers to make sure they are updated.

2) Change the Features Per Request from the default of 1000 to 100 or 200, so then less features are sent per request, and it should perform better.

3) Contact esri support and ask to be added to the following bug (I know it only says 503 and 502, but 50x errors are all related. BUG-000123780 : Intermittent 503 and 502 errors with 'query' and 'applyEdits' requests to ArcGIS Online hosted feature service

Check out this thread for more... https://knowledge.safe.com/questions/84687/arcgis-online-readerwriter-error-code-503.html