How to reproduce:
Open the dashboard and start navigating between tabs
The actual error:
XML5619: Incorrect document syntax. Line: 1, Column 1

Possible effects:
When using ajax calls without mentioning the accepted returned type may cause the object to be a string.

What I came up with in the research:
This was reproduced only on bulldog, not on my local env and not on EAC.
You can clearly see the the 'content-type' is returned in the network but for some reason being filtered from the value returned to the js code.

To make sure the issue is not a hacked jQuery version i loaded a fresh one:

$.getScript('https://code.jquery.com/jquery-1.7.2.js')

And used the new version to get something from server using $.get