Hi Supagoat thanks for the video, I did a closer examination and it looks like there may be an anomaly in the back inner curve here:



There are a couple of spots where the curve is making a sudden tight bend in shape, these spots:



It would probably be good to try and get the original curves to be a little simpler and smoother than this, without this type of sudden changes in shape in highly localized areas. That's not generally good for offsetting.

This is the spot that seems like it may be a problem especially:





You can see there where the curve's control points do an abrupt reversal of direction. I'm not entirely sure but it may be that the curve is back-tracking on itself or very close to back-tracking in that spot. I think that's what may be messing up the boolean.

One way to make this shape without doing a boolean is to do 2 lofts as you did, but with "Cap ends" turned off (unchecked) so the ends are open, and then select, then select all 4 curves and run Construct > Planar, that will build 2 end caps and now you've got 4 surfaces that can be glued together using Edit > Join. Doing a boolean will attempt to intersect surfaces with each other and that backtracking area is probably confusing the boolean.

But even with the non-boolean method that area may generate polygons that are back-tracking as well so it would be good to get the curves smoother and more simplified I'd think.

- Michael