Very many thanks. This is not a big problem, because it only occurs very occasionally - once every few days.
What happens is that the box fails to respond. I see a moving "Openvix" text at the top left corner of the screen, as if it is trying to perform a task, but has gotten stuck. I get it out of this state by turning the mains switch off and on so that it reboots. I usually leave the box on and sometimes it seems to have occurred spontaneously when I return after a few hours, even though it was working perfectly beforehand.
In future I will try to ascertain whether it happens only on a particular channel.