[Ichabod Fletchman]

Hi, I have the same box and have no problems with getting 3840x2160 at 60hz in GUI. Works just fine. So I would guess that it's an issue with AVR or TV. Because this has worked fine for some time for me.

EDIT: And I use the HDMI 2.0 port connected through a Marantz NR1606 AVR to a LG 65E6 TV.

If it is working for you, then I have hopes that I can do something to get it working.
But do you know why Windows 10 works at 3840x2160 60hz and LibreELEC doesn't?

It worked out of the box for me. I moved the SSD from a NUC6i5SYK to the NUC6i7KYK without any problems and the 3840x2160@60Hz was then set up as default.

I will check this a bit more when I get home from work.

I would appreciate that, but at the same time I hope you don't ruin your current setup. It would be tragic if you change a setting and ends up like mine.