it's a 206 http code which is just partial content delivered over SPDY aka HTTP2 these days. Where the package gets interrupted is hard to tell. Check your server logs it also could be you ISP, a proxy and so on.
All it says is that you didn't get the whole information. you could try and analyse it with wireshark I am not sure it is a good starting point I would rather check my server configurations if I have size / transfer limits by using smaller files etc.