[Andy]

All boolean objects must be 3D shapes, have their normals facing outwards and have no holes.

Instead of using a rectangle, try using a thin box.

The boolean functions are complicated and their efficiency can vary depending on the scale of the object. They area really designed for solid modelers, rather than polygon based modelers.

With the way booleans are currently implemented, re-booleaning a boolean result can cause problems.

If you send us a .ac file which causes the crash, we'll investigate.