1. increase your session timeouts in vbulletin settings..

2. inform your members to use the remember me box when they log in..
Log out and then back in with the remember me checked..

3. inform your members not to use the refresh or back buttons in their browser to start a new game..

4. inform your members to only play 1 game at a time..

5. Enable debug-mode, open /arcade.php and set $DEBUGMODE to value 2 (right at the beginning of the script)
Errormessage has extended information when set to 2..You can post the error messages here..

6. Do you have any kind of cookie-blocker ??