congratulations on having your D750 back! What was wrong with her?
Thank you. The shutter button had unpleasant and unusual friction. I found this symptom from the day I had bought it brand new. And then the button got stuck and the camear was frozen. Removing and inserting the battery got the camera back to work, but I decided to give it to repair.
Now the shutter button feels as smooth as it should.
The camera was, of course, repaired under warranty.