Voronoi Cube - "Boolean Difference set is empty" problem

Hello there,

I am an absolute GH newbie, and I tried to play around with a voronoi cube for some days now.

The basic stuff works so far, and I want to thicken my cube with weaverbird.

When I substract the surfaces it says" boolean difference set is empty", but I somehow get a result.

Nevertheless I can't join or thicken the resulting surfaces with weaverbird (says " topv_count is 2 or less")

So what am I doing wrong ?