Via Nikon Support Page:
"To users of the Nikon D750 digital SLR camera
Thank you for choosing Nikon for your photographic needs.
On December 29, 2014, we announced that we were looking into measures to address the issue reported by some users, namely that when photographing scenes in which an extremely bright light source, such as the sun or high-intensity lighting, is positioned near the top edge of the frame, flare with an unnatural shape sometimes occurs in images captured with the D750 digital SLR camera.
To correct this issue, Nikon will inspect and service, at no cost, the camera’s light-shielding components and adjust the AF sensor position. We plan to initiate this service at the end of January and will announce further details, including instructions for requesting servicing, shortly."
They say you can give them a call at 1-800-Nikon US (1-800-645-6687) for support.