Oliver,
Your elevation data was in there right, but you also had some other features with 0 or other elevations that was causing what you were seeing. To fix this, go to the Vector Display tab of the Configuration dialog and turn off point and line display, then generate your elevation grid (make sure to check the 'Flatten Areas' option and also the one to ignore zero elevations). Once that is done you can turn your points and lines back on.
Let me know if I can be of further assistance.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com