This is related to bug #65179 and the creation of arrays of classdef objects.  Making this report dependent on bug #65179.

The error message is now:

warning: struct: converting a classdef object into a struct overrides the access restrictions defined for properties. All properties are returned, including private and protected ones.
warning: struct: converting a classdef object into a struct overrides the access restrictions defined for properties. All properties are returned, including private and protected ones.
error: octave_base_value::parent_class_name_list(): wrong type argument 'object'

A way around is:

a(1) = foo();
a(2) = foo()
a =

<object array foo>

Concatenation of classdef objects fail with the error messages:

error: invalid conversion of multi-dimensional struct to scalar struct
error: octave_base_value::parent_class_name_list(): wrong type argument 'object'

It seems at least one part of the problem are the '__parent_classes__' and 'parent_class_names' functions which do not seem to support classdef objects.

To test, create a class, for example:

classdef foo
  properties
    bar = 1;
  end
end

and try to concatenate some instances:

a = foo();
[ a a ]
__parent_classes__(a)

If this isn't supported yet maybe a better error message should be printed instead.