Try doing this:
Be sure to play in Windowed Mode (not Full screen).
When you want to go on Forum or switch between characters on different accounts/clients, be sure to first Minimize the current Lineage 2 window and then open a new one. Always make sure you always have only 1 window maximized. This way you will never get critical errors. It always works for me. I bet there are other solutions though. 😛