In the first video it appears the second sketch is not closed.

In the second video: The centerline rail must intersect the plane of the section sketches or pass beyond. It can't stop short.

Make sure the centerline sketch geometry is perfectly smooth. ALL intersections must be Tangent or Smooth (G2). There can be no intersection that is even the slightest amount non-tangent.

Non-tangent intersections behave like a zero radius curve. This could cause the loft to self intersect which is not allowed.

If you could share your model it would be most helpful.