This the problem?
"We have received a few reports from some users of the Nikon D810 digital SLR camera indicating that noise (bright spots) are sometimes noticeable in long exposures, and in some images captured at an Image area setting of 1.2× (30×20).
After looking into the matter, we have determined that some noise (bright spots) may on occasion be noticeable when shooting long exposures, and in images captured at an Image area setting of 1.2× (30×20).
Nikon service centers will service these cameras that have already been purchased as needed free of charge to the customer. We sincerely apologize for any inconvenience this issue may have caused."
Sorry to hear it whatever it is Andy. Hopefully you will get a quick turnaround.