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|
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! :-)