DM500S - Virgin possibly causing problem

I have several DM500S users on my server running smoothly for a year. However, a couple of days ago 2 of them suddenly stopped working. I tried the boxes elsewhere and they're working fine. The routers are coming up with illegal DHCP errors. The 2 customers are both on Virgin Broadband. Their internet is working perfectly, connection to the boxes working but no channels apart from free to air.
I also have a couple of mates on someone else's server and they have the same problem the last few days.
I would add that I have other Virgin Broadband users on my server with no problems.
Can anyone offer any advice please?
Thanks