Why does the connection in the browser with the site via HTTPS break when submitting data from any form later than 5 seconds after the page is refreshed?

Question

iOS

I have a website running over HTTPS.
Everything works flawlessly from the computer.
If you access it from an iPhone, it also works great, but up to exactly one moment: if you open a page that has a form for sending any data, and send data through it later than 5 seconds, then a blank page will open in the browser with error: "Safari cannot open the page because the network connection was interrupted"

I use FirstVDS.ru services
OS: CentOS 7
Panel: ISPmanager 5 Lite
All default settings.

Tell me where to dig and who faced this?

Answer 1

digging into tracing, analyzing headers and traffic,
you can install an "accelerator" that runs something through Apple's servers, or something else, who knows

Answer 2

The other day I changed CentOS 7 to Debian 8 and the problem is gone...
Apparently, CentOS 7 is somehow configured by default that such a problem appears. I won't guess :)