"Constraint goal not reached" when reproducing manually an animation

Starting from

when I create an animation to move down the control point at the top, I get

If I manually move the control point, I get:

What’s wrong ?

Here is the Rhino file 2014-09-17 McNeel CLP 1250 cylinder.3dm (1.2 MB)

This file is foreseen to be without animation inside. It will be a block (with Bongo IK) inside a gantry crane file.

The object called “curve base” has the telescopic joint set to the X -axis when it should be set to Z.

The base indeed moves on rails following X direction.
The curve base (related to its parent point base) is free to move on x axis and must not move on z axis.
When I have saved the file, I had moved down the point ‘Cylinders top’. This point should be moved at its top position before to animate.