When the Fly proxy sends requests to your app, it will wait 60 seconds for a response. After that, you’ll see this error. Your app could be timing out for a number of reasons, such as slow database requests, or timeouts to other services that your app connects to.
If your application isn’t logging anything about these timeouts, check to make sure the clients for your target services (Postgres, etc) are configure to log when connection errors occur.