This has been partly mentioned above.

Let "a" be a number representing a coordinate.

The _Elastica2 script displays an error message, in all 4 views, for a center point with ( x, y, z ) coordinates ( a, -a, 0 ) or ( -a, a, 0 ).

The error message in MoI3 shows a line in the Script section of the _Elastica2.htm file:

var Frame = moi.vectorMath.createFrame( FrameOrigin, FrameXAxis, FrameYAxis );

The error message in MoI4, or MoI5beta, is the previously posted:
Script error
Invalid function argument
line 0
OK button.

- Brian
For nonzero z, coordinates ( a, -a, z ) or ( -a, a, z ) creates a curve, but with the strange angle mirror2 bug.

Will try some different code tonight or tomorrow, with better understanding of cplanes and Frames.