Just received this message(Attachment) which I've never seen before until now during the install/update of the Addon Manager.
What does it mean?
The solution to this problem, which is a XML file corruption caused by another defective 3rd party product (it was confirmed an OUTDATED version of Active Sky caused this problem, but there might be others around doing the same), is to open the XML file using a text editor that lets you change the file encoding, change to ANSI, and save it back.
An example of an editor that can do this, is the free Notepad++
Of course, any time you edit an XML file manually, you must also ensure your are saving it in the correct format, according to its header so, if it's like the default ( Windows-1252 ), you must save it in ANSI.