As to why it sometimes works with MOI's output and sometimes not, it's hard for me to speculate on that since the error is happening inside the pipe bender software.

Have you sent in a file that doesn't work to their tech support to see if it's something they can tune up?

If they require their STEP input to only contain analytic torus and cylinder entities, you'll probably need to use SolidWorks to output that since MOI isn't able to generate that particular type of restricted output right now.

One thing I don't get is why is it using STEP at all instead of taking in the DXF ?

- Michael