Well, it looks like I fixed it after some tinkering:
Longer story is that I had tried all these sweep options on the original geometry (thickness = 0.25), and all resulted in skewed mesh. I then scaled the geometry by 4x (thickness = 1.0) and just tried the last scheme I used on the original geometry (the “Advanced” option). Seeing that it was still skewed I submitted the above question. Only later did I go back through every mesh option to test.
Just as a side note, Greg, translation of swept solids to U-splines does not support many to one swept meshes, as they cannot in general be represented as a tensor product between a 1d and a 2d spline. You will need to combine those source surfaces into one virtual surface after they’ve been meshed but before the sweep is performed if you’re planning on building a U-spline on that mesh.
Greg, as of yet we haven’t been creasing the U-splines that we create from swept solids to ensure that they are locally linearly independent. I’m putting in a fix for that, which helps with this error; now instead of simply crashing the program, it fails to build the U-spline in the U-spline algorithm. I will pass this on to @luke to take a look at why that is happening and what can be done to fix it.