Hi wim, things that have that odd kind of display with a few polygons missing in the middle of the surface usually means that there's been some bug or issue (like self-intersection sometimes) with the trimming boundaries that have been created, with the UV space trimming boundaries being messed up in some way.

I've recently fixed up a similar bug for the next v3 beta but it does not appear to have fixed your case here.

I'll take a look at getting it fixed up, but when you have difficulties like this that's when some of the low level operations that I was describing before can come in for the rescue since sometimes they can get the job done where the higher level tool that bundles up a bunch of operations gets confused.

In this case the low level workaround is to use Trim followed by Join (for Trim select both objects, then run Trim and at the prompt to select cutting objects right-click or push Done right there which signals a "mutual trim", then switch mode = Keep and click on the 2 outside pieces to keep and then finish the Trim). Or also if you rotate the revovled shape around 180 degrees so its seam edge is on the other side that will avoid the problem for now as well.

At first glance I don't see any reason why this should have any problems though so I'll put it on my list to investigate and see if it can be fixed up.

- Michael