Now it works! The solution is, that the direction of the tangents at the start
and end control point of joining curves must be identical.
(http://wiki.povray.org/content/HowTo:Use_Splines_and_Bezier_Curves)

With this example it works:
prism {
    bezier_spline
    0, 10, 16
    <0, 1>, <.5, 1>, <1, .5>, <1, 0>
    <1, 0>, <1, -.5>, <.5, -1>, <0, -1>
    <0, -1>, <-.5, -1>, <-1, -.5>, <-1, 0>
    <-1, 0>, <-1, .5>, <-.5, 1>, <0, 1>
}

But now is the question, how can I make edges in the spline?

Thank you very much jr!

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