I don't use sweeps with guide curves like ever. Preference and lack of experience for that quite possibly, so I tend to favor other features to get the job done. I had some problem opening your file. Below is a general outline of two easy ways to do this. The file is attached with configurations for each option.
Looking at your part, it looks like you want a constant wall thickness it the center portion of this part, right? So split the part into a few bodies, shell the center portion to the desired wall thickness, combine the bodies, done.
Or create another sketch at the end of the guide curves. Use the loft or Boundary feature with the circles as the profiles or start/end, use the guide curves as guide curves, done.
Reducer.SLDPRT.zip 312.9 KB
Thanks for throwing some options for me as well. I found the issue on my model, just some relations that is affecting the model.