As per the error "backend_connection_closed_before_data_sent_to_client", the backend unexpectedly closed its connection to the load balancer before the response was proxied to the client. Please, follow this link for more information .
The GFE generates this HTTP response code when the connection to the backend is dropped during a request.
It is usually caused because the keepalive configuration parameter for the web server software running on the backend instance is less than the fixed (10 minute) keepalive (HTTP idle) timeout of the GFE. There are some situations where the backend may close a connection too soon while the GFE is still sending the HTTP request.
The solution for this issue, is to configure the keepalive timeout parameter for your web server appropriately. (In short, set Keepalive to greater than 10 minutes, such as the recommended value of 620.)
For more information, see the reference Google documentation below in link.
If it keeps failing after you changed the timeout, I’d suggest you contact Support.
 502s - "backend_connection_closed_before_data_sent_to_client" responses