Hi jernej !
I tested the last nightly image and the image is not working. I always have a Kernel Panic. So I tested the nightly image I download may 1st and I have exaclty the same problem, no image on my 4k tv but work well on my 1080p monitor.
I did another test, habitually everything pass through my Denon AVR, I always test my SBCs like that without problem. This time I tested to plug my H6 directly to my TV without passing through my AVR and everything work. I will continue to test your next images to see if the problem disappear.
I have buy this device only to have fun so it's not a problem to wait until all work correctly. Let me know if I can help you with some tests.