Hi Pilou, the curves must be able to be organized into 2 different sets that cross each other like a grid.
Imagine that each of the 2 sets will be lofted.
In this case these curves are your problem:
Imagine lofting just those 2 curves together, that will not work very well.
But if you split them up where they intersect, to make 4 different pieces, then you can see that lofting those 4 pieces will work. To split them at their common intersection, select both of them, run Edit/Trim, push Done to do a "mutual Trim", and push Done again to keep all the pieces.
That's what I did with the attached gordon_folly_fixed.3dm which will now work with Network surface.
Basically the curve network needs to form a 2D grid layout. It is ok if the 2D grid has one end pinched into a common point, or if it wraps around like a cylinder.
 Michael
