I would use a "Fill Surface" here.
For your problem,Did you try flipping the connectors?
Right click on the curve and you should see the option.
I'm pretty sure this is caused by the input curves/surfaces not having matching directions. If the input curves have matching directions the boundary command will automatically map the surface to the correct end points. If the input curve directions do not match we can override the default settings and flip the connectors to non-matching directions. Unfortunately sometimes the connectors will flip again during a rebuild causing the surface to fail.
Also unfortunately we don't have control over curve and surface direction inside SWX. A more robust way to fix this would be to match input curve and surface directions prior to the running the boundary surface command, but as I said, we don't have control over this inside SWX