Are you using a BT Homehub Router, if so you need to disable the smart setup on the Router (see info linked below how to switch off the smart setup on a BT Router).
If you are using a different type of Router then disregard my post.
Also what does the error message say, does it mention something about maximum allowed devices ?
What does the error message say ?
Also, there might be something on this link that could point you in the right direction.