When XPS Viewer is installed, it adds filters to the system to view and render .xps files. No time was put into making a graphical interface for XPS... so there is no "XPS-viewer". Instead the files are linked to the default browser (supposed to be IE) and IE loads the filter/drivers and renders the file.
The problem happens when a second browser is installed and takes the default browser setting. Then "XPS-viewer" settings passes the file to the new browser which does not load the drivers and handle it.
So, change the default application from "XPS Viewer" to Internet Explorer. That is the expected behavior and corrects the problem.
File Properties -> General Tab ->"Opens With", Change button. Set the application to Internet Explorer and it will open the XPS files.