That's a new one. It's not the same as a 500 error. A 500 error is sent by the webserver, and indicates that the PHP engine crashed when running the script. However, your error is not from the webserver level, but from the networking level. It indicates that something in between you and the webserver terminated the network connection.
It's most likely that the web hosting company has some proxy setup that is doing that; but it could alternatively be at your end. You should test it from different network locations (i.e. a different computer on a different network), to try to find the commonality. If it is at the web hosting company's end, then you should request support from them.
It would also be worth asking them where your PHP error logs are, and reading those. It's possible that, despite what I said above, in their setup a PHP error triggers a network reset.