So, we managed to solve it by modifying script.js and adding this line in a bunch of functions.
For the Quote button
function getXMLDocument(sUrl, funcCallback)
{
var oMyDoc = new XMLHttpRequest(),
bAsync = typeof(funcCallback) !== 'undefined',
oCaller = this;
oMyDoc.overrideMimeType('text/xml');
Quick Edit
function sendXMLDocument(sUrl, sContent, funcCallback)
{
var oSendDoc = new window.XMLHttpRequest(),
oCaller = this;
oSendDoc.overrideMimeType('text/xml');
Forgot to tell that the "Mark all mex as read" at the bottom shows issues too (it works but the "Loading..." bar on top never goes away). What should I modify?