If user experiences system reacting like - game page/landing opens, user can enter nickname and password and then after choosing server system redirect him back to game start page then most probably this user's IP changes too fast and system assumes it as a new connection.
Most often it happens while using a mobile phone and your ip address will change each time you connect to the internet. Even if you use a modem which has a sim or rim inside it - you will face the same problem. Connecting on the lease time of your mobile internet provider determines how often your IP address will change.

Possible solutions:
User could ask his internet provider for static IP address or use proxy server.

Game developers can do absolutely nothing about, so please, try the seggested solution.