I'm using Ubuntu 18.04 with a Zotac ZBox with Intel graphics 620 and a BenQ monitor connected via HDMI. The problem is that Windows has no problem displaying the full resolution, but Ubuntu fails if live-booted or after installation. There is no image, the monitor goes to sleep after a few seconds. It seems it cannot process the signal.

If I switch the resolution to 1920x1080, it works on Ubuntu without problems, so I suspect a problem with the signal for higher resolutions. However, I am a bit of a loss where the problem lies: On a Samsung TV, I do get a UHD resolution properly, but I also get UHD on the BenQ monitor with Ubuntu on another PC with an nVidia card. But the combination ZBOX (Intel), BenQ and Ubuntu does not work with the native UHD resolution.

I already tried adding a new mode to xrandr after running cvt 3840 2160.

What is a bit strange, so I mention it: If I run xrandr -q I see that the ZBOX lists DP-1 as name of the connected HDMI port, but it also has DP-2 for a displayport on the device, and a device HDMI-1 which is never connected. BTW, the displayport DP-2 does not have the problems I described. So maybe Zotac has done something funny to get HDMI 2.0 on the device via a displayport interface on the board, but it seems to work with a Samsung TV.

Does anyone have a tip on how I should proceed to track this down? What is the source of the problem, Intel, BenQ, Zotac, Ubuntu?

# lshw -c video
       description: VGA compatible controller
       product: UHD Graphics 620
       vendor: Intel Corporation
       physical id: 2
       bus info: pci@0000:00:02.0
       version: 07
       width: 64 bits
       clock: 33MHz
       capabilities: pciexpress msi pm vga_controller bus_master cap_list rom
       configuration: driver=i915 latency=0
       resources: irq:140 memory:de000000-deffffff memory:c0000000-cfffffff ioport:f000(size=64) memory:c0000-dffff

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