IE9 XMLHTTPRequest request body incomplete, but readyState = 4 statuscode = 200

I have a problem in IE9 where the XMLHTTPRequest request body is incomplete.

The code that processes XMLHTTPRequest waits for readinessState = 4, then checks statuscode = 200. Then it goes to JSON.parse, but it does not execute because the request body is incomplete. When I check the body of the request, I can clearly see that it is interrupted at some point in the line, for example, in the middle of the property.

There is very little to check on the server side, this is just a question echo json_encode($reply);

I know that AJAX errors can come from a wide range of sources, such as: excessive firewalls, but this will be accompanied by the corresponding status (for example, 403). In this case, the XMLHTTPRequest answer seems to believe that everything is in order, so I don't know how to detect it. Is there another property that I can check, or IE9 may have disabled the long response bodies?

I will continue to investigate this, but since I cannot reproduce it myself, it may take a couple of days.

Can anyone confirm these problems and perhaps point me to a solution?

Note. . This is similar to IE9, I believe that since older versions do not use XMLHTTPRequest, but ActiveXObject. This does not happen in Chrome or Firefox, at least: I have not seen it.