Of course, another useless IE error description, "System error: -1072896658"
This error is a result of the response's charset not matching what the XHR object is expecting (UTF-8). Obviously, this can occur due to numerous reasons. In my case, I had switched the server side encoding.From combing google results for this, I also saw that people had this issue due to specifying what they thought was UTF-8 encoding in an incorrect way, most proably resulting in the server encoding in it's default format.
To summarize, just make sure that your encoding is UTF-8 when using the IE XHR object. If you cannot do this, then there are other solutions, but none quite as easy as this.