Gigabyte Brix N3150 no Signal on HDMI

Hi everybody!
I recently got a Gigabyte Brix N3150 (N3150 Celeron, 4GB RAM) and now i’m trying to get my 4k screen working on the HDMI-port.
I have a 1080p on VGA and a 4k on HDMI connected.
Strange thing is: it gets recognized and all (brand-name, HZ, resolution…). I can enable/disable it under displays but all it says is: “No Signal” onscreen.
Tested HDMI on the 1080p and it worked.
Tested it under Win 10 and there both displays work: one in 1080p and the other in 4k.
EDIT: and tested Lubuntu and Mint from USB-Stick kernel which didn’t worked.