Hello Jens ,
To add what Adie explain ,please refer the below behavior.
For the Schedule base Contract->Item , the invoicing is reference to 'Schedule Invoice Request'(not with the reference to 'Customer Contract Invoice Request' ,which gets created on releasing of the Contract).
Once the Date is reach (within 3 days period), it creates the 'Schedule Invoice Request' under Invoice Request view with the status as 'To be Invoice'. On invoicing this 'Schedule Invoice Request' the status will further change to 'Invoice'.
Note : There exist NO status as 'Open' for 'Schedule Invoice Request' under Invoice Request view.
Best Regards,
Ankush Pashine.