Make sure you press the EXIT button at the end of your session.
If you see the following message when trying to log in using Java:
You are already logged on using a different computer, please leave the club with the other computer first!
then you closed the web browser, but forgot to EXIT.
Enter the club again using the NeXtGen and press EXIT this time.
I keep trying to enter by Java and am told that I cannot because I am already logged on on a different computer.