It may just be a minor lens contact problem. Try using a bit of 90% or stronger isopropyl alcohol on a swab to clean the contacts on the end of the lens. There might be just enough oxidation to fool the D800 for some reason, but not your D300. It happens. Also, make sure the D800 body mount lens contacts are clean and unimpeded.
BTW, use only an alcohol-damp swab, not a wet dripping swab. Make sure you don't leave any cotton swab bits behind. Only takes a few seconds to clean the contacts and it might do the trick for you.
Do not use rubbing alcohol. It contains a bit of oil which will leave a transparent film behind and impede contact. Pure isopropyl only - 90% or higher. Do not use electrical spray degreaser or spray contact cleaner anywhere near a camera or lens unless you spray it on a swab (well away from your gear) and then use the swab on the electrical lens or body contacts. Spraying any sort of electrical cleaning or degreasing compound of any kind in the vicinity of a lens coating or open camera body is a very b-a-a-a-a-d idea.