I was mostly talking about the own ship pointer.
Obviously the dashboard displays give their respective headings, but for example when the ship is moored, the pointer (and the vector) was just going around in circles.
But as is always the case with these things, I just booted up everything again and now suddenly the right heading is being used. Could be related to which instrument's data OpenCPN receives first maybe? I'll try next time it's not working
Thanks for the thinking anyway.
PS: the correct sentence was $IIHDG,250.3,,,,*4E