Office Excel Macro doesn't work correctly with NetWorkDays function

We use the NetWorkDays function often to know how many work days do we have in real.
But when we would like to show the Excel file with the Office Connector it shows the wrong day count, if we do not include the weekends.
And this won't work always, too.
If you open the Excel file with MS Excel the counts are right.

Find attached the example Excel file and a screenshot of our test in Confluence.
You can work with the Excel file in your test system.

We gave the prio "Major", because we use this function often in our Excel files.