Hi!
Today my laptop ran out of battery. I could not reconnect with my
nick, because it was still present on the server. Then I reestablished
my internet connection. The nick was still there. I had to log in with
an other nick.
It was still present in an open game, and I think, for that the game
admin could not launch the game.
After about 10 minutes, the name disappeared, and I could reconnect with
my old one.
Maybe the server should check active connections more often, and kick
inactive users.

There is no mechanism in TCP to "immediately" detect dead connections, and PokerTH does not implement additional ping mechanisms, it only times out gracefully. A solution is to use SCTP instead of TCP, if your operating system supports this, but to do this you need to set up an IPv6 tunnel.