Problem with the "invalid url to installer files" error while attempting a rolling pool upgrade. The web server was IIS 8. The problem was that a mime type for the empty extension was not defined. Solved by creating a new mime type of "." [actually type a period and nothing else into the text box] with type "application/octet-stream". You also need to have a mime type for bz2 and possibly more. I simply created a mime type of "application/octet-stream" for extensions ".*" to include all corner cases. This resolved the 404 error and Xen Center could now see the install files on the website.
