Install Zebra Setup Utilities.
https://www.zebra.com/us/en/support-downloads/printer-software/printer-setup-utilities.html
Select 'Configure printer settings'.
For the correct size (for PostNL label 102 x 150mm) and select 'Finish'.
Now select 'Open printer tools' -> Action -> Calibrate media (2 to 5 labels will be printed)
The tool can now be closed and the label printing should work correctly.