Failed to load resource: Request timed out on Safari

We have a web application working correctly for over a year now on most browsers. Recently we discovered it is not working that well on Safari.

A lot of actions end up with the following error : Failed to load resource: Request timed out. Funny thing is the action is actually performed correctly after that (most of the time).

When looking into the error, it seems to happen when there is an ajax request.

First I tried to change the ajax timeout setting by doing the following :

 $.ajax({
      "type"      : methode,
      "dataType"  : "json",
      "url"       : url,
      "async"     : async,
      "data"      : donneesEnvoyees,
      "timeout"   : 60000
 })

That didn't change anything at all, error is actually showing up after about 10 seconds which is less than the timeout defined.

After reading a bit on the internet, I saw some answer about specifying no-cache so that safari doesn't keep the post parameters in cache. I cannot say I fully understand that, but I still tried the following way :

$.ajax({
     "type"      : methode,
     "headers"   : { "cache-control": "no-cache" }, <-- added this line
     "dataType"  : "json",
     "url"       : url,
     "async"     : async,
     "data"      : donneesEnvoyees,
     "timeout"   : 60000
 })

As you can guess, I still get the same error.

Do you have any idea of what is happening? Why is this error happening only on Safari and not other browsers? How to fix it?