"PowerPivot is unable to load the Data Model"

I have an Excel workbook with sheets with PivotTables using PowerPivot to fill them. The source are SQL Server views and a stored procedure. Somehow I receive now an error message:

- An issue with the Data Model is preventing Microsoft Excel from opening this workbook. Try restarting Microsoft Excel.

followed by

- PowerPivot is unable to load the Data Model

I have Windows 10 and Office 2016. The first message I can't find on the internet. On the second one I found several tips (repair Office, reboot, etc), but nothing helps. Also on a colleagues computer this problem occurs (so it is in the workbook itself). An old version from 2 weeks ago, still works.

Any idea? Any suggestion?

Thanks in advance,

Regards Gerben