I would think there is a few issues here that are mesh related.
- Edge density is at ~650 edges which very high.
- The boundary has tons of edges in areas that are really not needed
My approaching to fixing things would be to use Curvizard. Cleanup and resample the curve to re-create the shape with a more manageable edge count. Without affecting the silhouette.
You could go from ~650 edges to <300 quite comfortably.

If you overlap the profiles you can see no benefit from the excessive edge count.
You can then run Fredo Corner:

I would guess the reason RoundCorner 'works' is because Fredo Corner has parametric abilities to restore or edit bevel post-op. If the input is troublesome then Fredo Corner is saying 'I can't do this because the computation and parametric abilities are not suited to geometry'.

