I don’t really known it’s a bug or not, but I’m experiencing this problem which is word 2013 always asking for user credential to access proxy officeimg.vo.msecnd.net.
I can’t continue to use my word application until it’s pass the authentication which is I don’t have.
After I’m googling around, I manage to found workaround to this issue.
Just unthick this property on: File>options>trust centre>trust options>privacy> “Allow office to connect to the internet”.
For more information, check this link here.