Here are the general guidelines i use to prevent this error 99% of the time on my board.

1. increase your session timeouts in vbulletin settings (around 30 mins is good unless a game takes longer to play)
2. make sure holdsession.php is uploaded and in the right place.
3. inform your members to use the remember me box when they log in
4. inform your members not to use the refresh or back buttons in their browser to start a new game.
5. inform your members to only play 1 game at a time

i used to get nothing but complaints of the same error, but since following these guidelines the error has all but disappeared.

Server-Timeout (seconds) for Score-Submission
After X seconds gameresults get a timeout when transmitted from the game to the arcade.
The lower the better (more secure), depends on your serverspeed (Default: 3 seconds)
If users complain about timeout-errors, increase this value a little