That URL looks fine. I don't manipulate or change the URL generated by Telerik's ReportViewer in any way. If you copy that URL, paste it into a new browser window and get the report, you know it's fine. Are you using the native XMLHttpRequest object directly? What does your code for creating and sending the XHR look like? Is your response actually null inside the `load` event handler?
Before I converted to Angular, my code for creating the XHR object and sending it looked like this:
01.function windowOpenWithXhr (url) {
02. var xhr = new XMLHttpRequest();
03.
04. xhr.open('GET', url);
05.
06. xhr.addEventListener('load', function () {
07. // handle the response here
08. // inspect the 'this' object to view the response
09. }, false);
10.
11. xhr.setRequestHeader('myCustomHeader', 'myCustomHeaderValue');
12. xhr.responseType = 'blob';
13. xhr.overrideMimeType('application/octet-stream; charset=x-user-defined;');
14. xhr.send(null);
15.}