There’s a little more information here Router errors 502 of POST requests "Undocumented"
What’s the fly status on the app? It’s useful to know how many instances are running and where they are.
In this particular case, it seems possible that the connection was cut ( the umbrella / catch all error.code=1) as the app went down / was terminated for whatever reason (fly checks list should tell you which health checks are failing). There seem to have been no good instances available on the next couple of requests (error.code=1002).