Random "net :: ERR_CONNECTION_RESET", "XMLHttpRequest: Network error 0x2eff"

I have a Symfony 2.8 application.

Everything worked well with PHP 5.6 and Apache 2.4, on Windows 10. Then I upgraded to PHP 7 (and kept Apache 2.4).

As soon as I upgrade, I start to get errors randomly:

"net::ERR_CONNECTION_RESET"

in Chrome

"XMLHttpRequest: Network Error 0x2eff, Could not complete the operation due to error 00002eff."

in IE 11.

When I mean random, I mean I reload the same page, with the same content, and sometimes I get, sometimes I don’t. I find 5 or 6 Ajax calls on this page. This usually happens on these calls, but sometimes it also happens on the main request.

I have not changed the Apache configuration (other than pointing to the correct PHP version) and have made a diff for php.ini to make sure all the options are the same.

I've already read here to no avail.

I've tried KeepAlive off

as well:

KeepAlive On
KeepAliveTimeout 5

      

        
        
        
      

    

and

KeepAliveTimeout 60

      

        
        
        
      

    

I've also tried: SSLVerifyCLient optional_no_ca

So to wrap up: this is not a matter of content, not browser dependent.

Also, it doesn't seem like something to do with synchronization: Ajax calls take 4-5 seconds to complete (but I've seen them fail even when they take 1 second).

One of my colleagues is experiencing the same problem (Apache 2.4, PHP 7 on Linux), but the other is not working (Apache 2.4, PHP 7 on Linux, but php works as an Apache module).

Apache log is clean as well.

Any hint?