@Monkey Frog: With physics off on an object isimmobile doesn't actually do anything.
For a physics object it does two things, 1) Ignores the Bullet engine position updates for that object and 2) allows a dynamic object to have polygon collision shape. Obviously with physics off there is no physics object at all.
My guess in this case is that both the locker and the door are physics objects and their shapes are overlapping causing the engine some grief, by turning physics shape off for the door they will be able to coexist happily together. The player will not really notice as the collision shape of the locker will still be there.
It will mean that the player would then be able to walk through the door itself when open but that is such a common occurrence in 3D games I doubt anyone will gripe about that.
If you want to go all out for realism then the proper way would be to create a physics hinge between the door and the locker and open/close it by controlling the hinge angle but there is no stock behaviour for that I believe.
Been there, done that, got all the T-Shirts!