This is probably because the cable is struggling to cope with the 4K 60Hz signal from the Xbox over that distance, whereas Sky and your UHD player will probably be using a lower refresh rate so you don't see the issue when using them. We had to replace our long HDMI from the amp to TV with an HDMI fibre cable as we were having black outs every now and then, but only on the PS4. Once we'd changed to fibre these issues went away. Before the new cable we thought fibre would be another case of the emperor's new clothes doused in snake oil but no!
However I'm prepared to be shot down if this is giving you a bum steer!
A fibre HDMI cable may not be quite as expensive as you're thinking - we bought a 15m version of this cable (and note that there's also an £3 discount tick box just below the price):