"ip address conflict with aother system on the network"

I've been getting a popup error message for about a week:
"There is an ip address conflict with aother system on the network". This is only for 2 desktop PCs with wired ethernet connections. Wireless devices are fine with no error message. The wired LAN PCs basically lose all network capability. Reboot fixed it, but it returned every time it was rebooted.

I have a Virgin Media Superhub with another router configured as an access point. To cut a long story short, after trying to reset everything, re-installing network adapters & swapping ports, I decided to set the Superhub to Modem Mode, and re-configure the access point router to act as a standard wireless router. In other words renewing the whole network's LAN IP addresses.

I now don't lose network capability, but the error message is still appearing on both wired LAN PCs. I use the Fing app on my android phone which lists IP addresses for all devices on the network, wired and wifi, and no 2 addresses are the same, which suggests that DNS is doing its job.

Any ideas why I'm getting this message?

Could this be connected with my WAN IP address or some external IP address? Should I be contacting Virgin Media about this?

Any help would be very much appreciated!!