PowerPivot in SharePoint 2013 - The schedule has been altered outside of PowerPivot. Use the Manage Data Refresh page to update and re-save the schedule

Ouch. This one sucked.  It sucks because the error you see in the PowerPivot refresh history doesn't tell you much.  Here's what we were seeing in the refresh history:

The schedule has been altered outside of PowerPivot. Use the Manage Data Refresh page to update and re-save the schedule.

We'd see this error any time we'd try to kick off a manual refresh.  Even more strangely, we'd see that the Excel file in question required a check-out before we could modify the refresh schedule.

Hmmmmmm....

So it turns out you need to disable the requirement for checking out a file for any Document Library in SharePoint 2013 where PowerPivot scheduled refreshes are enabled.  Don't believe me, check the TechNet article:

http://blogs.technet.com/b/excel_services__powerpivot_for_sharepoint_support_blog/archive/2014/09/15/powerpivot-scheduled-data-refresh-quot-the-workbook-must-be-checked-out-before-it-can-be-replaced-you-can-save-this-file-with-another-name-quot.aspx

Also - that's one of the shortest, most succinct TechNet articles I've ever read. 

At any rate, we were seeing the following in our ULS logs as well.  It was actually the ULS error that pointed me to the TechNet article.

EXCEPTION: System.InvalidOperationException: Call to Excel Services returned an error. ---> Microsoft.AnalysisServices.SPClient.Interfaces.ExcelServicesException: The workbook must be checked out before it can be replaced.  You can save this file with another name.

So the fix, in case you didn't go directly to the TechNet article, is to remove the requirement for check-out in the Document Library. 

I'm posting this one because when I searched on the initial error, I found one TechNet thread that didn't have the resolution I needed.  Hopefully this will help someone out.