According to Wikipedia, 4K at 60Hz is 14Gb/s and you are trying to drive that down a 5m cable. There's a whole bunch of signal integrity engineers who just curled up in to a little ball and started weeping after reading that! Slight variations in signal drive strength between the Xbox and the PC, or differences in trace layout between the connector and the chip will be enough to cause this kind of problem.
FYI, it's not necessarily the case of the Xbox being too weak to drive the cable. The signal drive strength must be carefully tuned to get a good eye diagram
at the receiver. If you drive the signals too hot then you get all kinds of nasty overshoot, ringing and other nasties.
I'm just a software developer but I've had tables of drive strength and sensitivity given to me by signal integrity engineers to program in to high speed SerDes
to get things to work.