What's this error?

This is a very common error usually caused by a core Roblox script (when player leaves, script malfunctions and error appears).

assetgame.roblox.com/game/gameserver.ashx

This is the link to which you're receiving the error from.

The good news is this script should never effect any of your client-side scripts because this is occurring in Roblox's script. It also won't effect much because since the player leaves the game, the script cannot follow up with any more errors.