the comment #0 code fails on line 108, which is:

u = (6*(1-p)*QT*diag(1 ./ w)*QT' + p*R) \ (QT*y);

 whos p QT w R y
Variables visible from the current scope:

   Attr Name        Size                     Bytes  Class
   ==== ====        ====                     =====  =====
    f   p           1x21                       168  double
        QT         19x21                      1088  double
    f   w          21x1                        168  double
        R          19x19                      1040  double
    f   y          21x1                        168  double

the QT*diag(1./w)*QT' part is fine, but then the (1-p)*ans part produces

error: operator *: nonconformant arguments (op1 is 1x21, op2 is 19x19)

looking at this, i realize it expects p to be a constant to set the smoothing parameter. Octave's implementation does not accept other types of this parameter.  from the matlab help:

"p — Smoothing parameter
scalar in the range [0,1] | vector | cell array | empty array

Smoothing parameter, specified as a scalar value between 0 and 1 or as a cell array of values for multivariate data. You can also specify values for the roughness measure weights λ by providing p as a vector. To provide roughness measure weights for multivariate data, use a cell array of vectors. If you provide an empty array, the function chooses a default value for p based on the data sites x and the default value of 1 for the roughness measure weight λ."


ok, so that at least narrows down these errors as being related to csaps not accepting the full range of input types now supported by Matlab's csaps. The lack of input type checking allows the inputs to propagate through and create the nonconformant arguments error messages

Retitling to better capture the issue, changed the category.

recommended short fix:  improve usability by adding input type checks and more useful error messages

recommended long fix:  implement the missing matlab compatible input handling options.

One of the matlab example scripts for the csaps function throws a nonconformant argument error when run in Octave 6.3.0, splines 1.3.4:

2nd example from https://www.mathworks.com/help/curvefit/csaps.html

x = linspace(0,2*pi,21); y = sin(x)+(rand(1,21)-.5)*.3;
pp = csaps(x,y,0.4,[],[ones(1,10),repmat(5,1,10), 0]);
pp1 = csaps(x,y, [.4,ones(1,10),repmat(.2,1,10)], [], ...
                    [ones(1,10), repmat(5,1,10), 0]);
x = linspace(0,2*pi,21); y = sin(x)+(rand(1,21)-.5)*.3;
pp = csaps(x,y,0.4,[],[ones(1,10),repmat(5,1,10), 0]);
pp1 = csaps(x,y, [.4,ones(1,10),repmat(.2,1,10)], [], [ones(1,10), repmat(5,1,10), 0]);
figure
hold on
fnplt(pp, 'b');
fnplt(pp1,'r--')
plot(x,y,'ok')
hold off
ylim([-1.5 1.5])
title(['Cubic smoothing spline, with right half treated differently'])
legend('pp Larger error weight', 'pp1 Larger error and smaller roughness weight')

results in the attached figure comparing pp to pp1

in Octave, I get:

pkg load splines
x = linspace(0,2*pi,21); y = sin(x)+(rand(1,21)-.5)*.3;
pp = csaps(x,y,0.4,[],[ones(1,10),repmat(5,1,10), 0]);
pp1 = csaps(x,y, [.4,ones(1,10),repmat(.2,1,10)], [], ...
                    [ones(1,10), repmat(5,1,10), 0]);

error: operator *: nonconformant arguments (op1 is 1x21, op2 is 19x21)
error: called from
    csaps at line 108 column 5