Hi Olga - it looks like this is due to some differing lengths between those 2 rails.

If you zoom in to the other non-rounded end of your sweep:



Notice how the initial profile between those 2 rails is quite slanted? One rail is sticking out more than the other one so the profile starts out slanted and continues slanted throughout the length of your sweeping, giving you that result that you don't want.

You might try trimming off the end of that longer part so that the profiles will be closer to starting more perpendicular to both rails rather than slanted between them.

Sweep also can currently be sensitive to the spacing between control points in the rail curves, which may introduce this kind of slanting as well. That is something that I plan to tune up in v2, but it kind of looks like in this case it is starting quite slanted at the beginning there due to the differing lengths so that is probably something to adjust first.

- Michael