Hi martingg,
Thanks for your post and apologies to hear the picture is not great.
Taking a look at your boxes, all 3 are looking good in terms of signal, power and input levels.
As this is happening on only 1 box, it would be interesting to see if the issue persists on the same box when swapping them over.
If it does then this would suggest the fault lies with the box itself.
Pop back and keep us posted on how you get on.
Thanks,