Going back and forth with Microsoft technical support right now over a SharePoint issue. Good Lord I want to reach across the wire and smack them in the face with a sea bass. Not enough to hurt, but get their attention and smell like fish for a while.

No genius, the warning on the PowerPivot Data Refresh page 'Warning: this page is not encrypted for secure communication ..' IS NOT the problem. The error messages I sent *three times* from the ULS logs are the symptoms you need to be researching. Stop guessing and trying to blame any random message you see on our configuration.