When doing a sweep, the sweep profile doesn't have to be centered on the sweep path.
In some cases, if the sweep profile has some extension on the inside of the curve, it will need to go around a turn with a radius tight enough that the inside portion of the profile creates a path radius less than 0, as in a negative radius, it will throw an error. Picture walking while carrying a long post over your shoulders. When you turn to the left, the portion of the post on the left side has to move backwards to accommodate the turn. Sorry, I may not be articulating this very well. Sometimes what makes sense in my head comes out as gibberish.
Another way to explain, picture the sweep profile traveling along the sweep path. When it makes a left turn, if the radius of the turn is so tight that the portion of the profile on the inside of the turn has to move backwards, it will throw this error.
One way to avoid it is to create your sweep path on the inside edge of the sweep profile. This way, there is never any portion of the sweep profile that self intersects along the path. Kinda like this: