Found the Problem:
My EarthRoamer was standing at the other side at the map at the warehouse platform.
This was the "construction area" called at the message at the port ... nothing to do with the place I wanna unload the garbage at the port.
It's a bit .... hmmm!
Mission "Warehous Problems"
Unable to unload the garbage at the port in the unload area.
Here comes the message "Please move your vehicle out of the contruction area"