Hi Paoki,
You will surely get a response from Michael, but I thought I could input also. If you do an op on a solid and it turns into a "joined surface. This script can help you find the culprit. It's called "get Naked Edges" and its a good help for this.

moi.geometryDatabase; gd.deselectAll(); var breps = gd.getObjects().getBreps(); for ( var i = 0; i < breps.length; ++i ) breps.item(i).getNakedEdges().setProperty( 'selected', true );

Also a temp help for you to find the break in your curve could be to launch the "extrude command and look at the suface popping out. There should be a line at the break points. You could then zoom to that area pretty quick with the zoom area tool.

Anyway.....2 Cents.