Power Pivot Error While Upgrading a Workbook

I recently tried upgrading a workbook to Excel 2013 from an older version (I believe 2010).

Upgrade message

It succeeded (or at least it looked like it succeeded), but I needed to save, close and reopen the workbook according to Excel. 

Upgrade Data Model

Close and reopen workbook

Saving of the document failed with the below error message and then the Power Pivot Add-in was corrupt and the whole tab was even gone.

Error message

Disabling/enabling the Add-in didn't work, I got an error message saying the add-in cannot be loaded and Office Repair needs to run.

Office (Pro Plus) Repair ran successfully, it only needed a reboot after which the problem was solved. I still have no idea what caused Excel to blow up like this.

The second try of upgrading the workbook did work correctly and now I could save and reopen it and go on with my business! :-)