A feature/bug in Safari for the Mac causes file downloads to fail for a user who is logged in. Here are two workarounds:

• Use another standards-compliant browser like Firefox or Chrome.
• Choose a different user agent within Safari. To do so
  • Go to the Advanced Preferences pane of Safari and select the "Show Develop" menu option.
  • This gives you another option in using Safari, labeled "Develop."
  • When you're on a web page in our system that has a link to a file, click "Develop," and then on the menu you get, click "User Agent." This gives a list of browser versions. Pick Firefox or IE. Then refresh the page.