thanks all, it works indeed when moving the tubes a little, they were intersecting indeed.
another 2 things learned about MoI today! (didn't know about the intersection and about how to move points of a nurbs volume).
I thought I knew almost everything by now, but still...
next month I will be officially teaching with MoI!

cheers
wim

So another thing to note here, is sometimes you don't want to, or cant start moving things around.

So with your example, you could cut the tube into 2 pieces (just before it starts to intersect) then union the 2 pieces together which will remove the intersecting surfaces and allow the Boolean to succeed.

Although, in this case, the intersection was so small and slivery, I had to do an interse3ction curves on the 2 pieces, then use "trim" to remove the little intersection surfaces and then join back to a solid and Boolean diff away from the block...

FYI.