It sounds like a problem that affected the very early KAT5 units with some makes of set top box.
the problem was that the set top box did not see a dc coupled 75 ohm termination as the termination was AC coupled.
A slight modification cured the problem in every case and became standard after the first 50 sets of Mk1 KAT5 units.
Your serial number indicated that yours is a Mk2 unit so should not suffer from the problem at all.
The one thing that could make it happen is if the termination switches were turned OFF in the Mk2 transmitter. This would cause the picture to be brighter than normal but mnost TVs would work OK, however a set top box thet required a DC termination wouldnt see one and could give rise to the problem you describe.
Check that all 4 of the termination switches inside the transmitter are turned on, it could be that the switch contact is slightly tarnished and not making good contact so try switching all 4 switches on and off a few times as this will clean the contacts.
Failing that, email me and I will check the unit out for you.