Nikon USA has issued a service advisory for the reflection/flare issue on the Nikon D750 camera. You can check you D750 serial number on this website (here is the EU link) if is affected and needs to be repaired. And if your camera already got repaired, you will see a black dot inside the tripod socket on the bottom of the camera.
Full Nikon D750 service advisory text:
Thank you for choosing Nikon for your photographic needs.
We have received indications from some users that when photographing scenes in which a bright light source, such as the sun or high-intensity lighting, is at a certain position along the top border of the frame, flare with an unnatural shape may sometimes occur in images captured with the D750 digital SLR camera.
For those who have already purchased a D750 and would like to have their camera serviced to correct this issue, Nikon service centers will service cameras as indicated below. We sincerely apologize for any inconvenience this issue may have caused.
Identifying affected products
To check whether or not your camera is one of those affected by this issue, click Check Your Serial Number below and enter your D750’s serial number as instructed. Results for the serial number entered will be displayed. If these results show that your camera is one of those affected, instructions for requesting service will be displayed. If your camera is not one of those affected, rest assured that you may continue using your camera without concern for this issue.
Check Your Serial Number
Please note that even if a camera’s serial number identifies it as one affected by this issue, a black dot inside the tripod socket on the bottom of the camera, like that shown in the photo below, indicates that the measures to resolve this issue as indicated below have already been applied.
If your camera is affected, as confirmed with serial number check above you will be provided with information on requesting service from your nearest Nikon service center. The service center will inspect and service light-shielding components, and adjust AF sensor position, to resolve the occurrence of unnaturally shaped flare free of charge. This service will not have any effect on autofocus precision, or on the performance of other camera functions.
Please understand that even with these measures, all digital SLR cameras may experience flare when backlit scenes are being captured.
We sincerely apologize for any inconvenience this matter may have caused you.
Please be assured that Nikon is continuously working and taking measures to further improve the quality of its products. Once again, thank you for choosing Nikon for your photographic needs.