We finally found the solution to this problem, our server returned text/json;charset=UTF-8 for JSON responses.
To fix the problem, open /framework/admin/javascript/LeftAndMain.js and replace the following line:
if(xhr.getResponseHeader('Content-Type') == 'text/json') {
With:
if(xhr.getResponseHeader('Content-Type').indexOf('text/json')===0) {