just flew my magic air 2 for the second time and after only 300 metres it starts to send
messages about weak rc signal. Full batterie's on both drone and controller.
Have tried several landings, restarts and so on but the problem stays.
What could be the problem ?

Hello there jos oosterhout. I am sorry for the trouble this ha caused and thank you for reaching out. Can you please fly your DJI Mavic Air 2 to a different location with less interference to see if the issue will still persist? The said DJI drone might have picked up magnetic interference from the area. That might be the reason why you are experiencing these issues. In addition, The signal between the drone and the remote controller is most reliable when the antennas are positioned in relation to the drone as depicted below. Thank you and fly safe always.