‘err’ usually means that the lens connection is loose or dirty, try cleaning the contacts on the lens and the battery pins… call Nikon if that doesn’t help, you shouldn’t have any problems with a more than one grand worth of camera.
Have had my D300 since xmas – over 11,000 images so far many on high burst ( bike outings) and no issue for me I’m afraid – I’ve used up to 8gig Extreme 4 cards