Game crashes when entering the nether, but not when leaving. It happens on 2 of my computers but not on one.

Like i said i enter the nether and the game crashes. When i log back on i am in the portal in the nether and am sent back to the main world. If i enter the portal again to the nether the game crashes again. So it looks like I can leave the nether but not enter it. Oh also this is a multiplayer game running on my computer. The game crashes on my computer and my sons but not my wifes.