This is the topview, as you can see there are no gaps between the walls.
The light goes THROUGH the wall and not through a gap.



And this is the frontview, same here, light goes THROUGH the wall and no gaps.
Snap in WED is on with a value of 1.



The walls are 8 quants thick. The only way I can prevend this to happen is to set the HiRes in the properties from the lightsource.
In Map Properties; Mesh size = 0.
And like I said this is NOT happening in version 6, so I think that Frank has a point here.