It seems that a lot of stuff changes with IE11. Some legacy code stopped working out the blue.
After some debugging it seems that some expressions that have been evaluating to true, now are being evaluated to false.
alert("YES!!!"); //this never gets called in IE 11
Boolean(document.ActiveXObject) gets evaluated to true in IE 10 but in IE 11 is evaluated to false, while it is still a valid function.
Whatever, just thought this might help someone.