I mentioned it a few pages ago, but it was this:
When I got the body back, I took a couple of images, but noticed that some settings had changed, so I went into the menus. I reset some things and then went to look at the firmware version and, at that point, the camera froze/locked. The only way out was to remove the battery. Once I'd done this and replaced the battery, the camera no longer 'saw' the memory cards, the rear screen wouldn't come on at all and the metering wasn't registering changes in brightness (plus some other issues). It was like this repeatedly for 4 or 5 days as I had an email conversation with Nikon and tried various things out
When they told me that they couldn't reproduce the fault, I asked them if the camera was working normally for them, but didn't get a direct answer. Only that they were still checking it. At the end, they said that the symptoms I had described were consistent with a problem with the main PCB and that they were going to replace it.
Another aspect that wasn't raised at all by Nikon (and has been discussed up thread) is that my D750 came from Panamoz so, strictly, hasn't got Nikon EU warranty cover. I'd sent it off with my proof of purchase and was ready to tell them that the problem must have be caused by the first service, but it never came up.
I've noticed that it has come back this time with the 'L' firmware value (distortion control lens data) showing as 2.005 (I think?) whereas, when it was returned before, it was 2.009. I'm loathe to update this now! What are other people using?