@vv You wrote:
"The strange fact is that any function of a complex number commutes (.) with any expression:
f(2+3*I).b - b.f(2+3*I) = 0. "

if f maps C into C then there is nothing strange about
f(2+3*I).b - b.f(2+3*I) = 0, whether or not b is a vector or a matrix.
The dot will be interpreted as multiplication by a scalar:

 

'(4+47*I).Vector([3,2])';
%;
'(4+47*I)*Vector([3,2])';
%;
'Vector([3,2]).(4+47*I)';
%;
'Vector([3,2])*(4+47*I)';
%;

It has always been lige that in Maple.
So the statement in the help page for dot:
If A and B are numbers (including complex and extended numerics such as infinity and undefined), then A . B = A*B.
ought to be modified to something like this:
If A or B is a number (including complex and extended numerics such as infinity and undefined), then Then the dot in A.B and B.A will be regarded as multiplication by a scalar

The help page continues with this:
 

If one of A and B is a Matrix or a Vector, and the other is a Matrix, Vector or constant and the previous case does not apply, then their product is computed by using LinearAlgebra[Multiply].  See also simplify/rtable. 

Here the case when one of A or B is a constant is covered by the the documentation for
LinearAlgebra[Multiply], but the present simple case drowns in all the cases that are handled.

@vv Here is a counterexample:
 

#f(2+3*I).b - b.f(2+3*I);
restart;
f:=x->x*Matrix([[1,2],[3,4]]);
b:=Matrix([[5,6],[7,8]]);
f(2+3*I).b - b.f(2+3*I);

The answer from the last line is:
Matrix([[-8 - 12*I, -24 - 36*I], [24 + 36*I, 8 + 12*I]]);

This is at least as old as Maple 8; I just checked.

The description in ?dot is basically the same as in Maple 2025.
The second line for example in Maple 2025 is

If A and B are numbers (including complex and extended numerics such as infinity and undefined), then A . B = A*B.

This is exactly the same as in Maple 8.

The behavior is indeed strange (at least somewhat).

@vv Here is an assuming version that works:

restart;

puzzle := sum(cos(k*Pi/(2*n + 1))^4, k = 1 .. n);

expand(convert(puzzle,exp));
simplify(%) assuming n::posint;

@vv This version works:

restart;
puzzle := sum(cos(k*Pi/(2*n + 1))^4, k = 1 .. n);
simplify(expand(convert(puzzle, exp)),assume=n::posint);

See simplify/details.

@janhardo I know how to make a screenshot.
I usethe special  PrtSc.

What I want removed is this whole thing:
 

@vv Thank you for the reference. Now i have Maple 2025.1.
I had tried Check for Updates in Maple 2025.0 a couple of times, but the answer was "No updates available".

@C_R After installing Maple 2025.1 I tried my worksheet made in 2025.0.

As you said it still doesn't work on that worksheet.

Then I tried copying the input lines and put them in a new fresh worksheet. Then both computations could be stopped.

@vv I'm not aware of the existence of Maple 2025.1. 
Maple 2025.0, however, cannot stop ifactor(x).
Maple 2024.2 can be stopped without problem.

So I certainly hope and expect that when the update 2025.1 comes it can stop that calculation.
 

@WD0HHU Are you sure you are seeing the whole bottom?

Take a look at these two screenshots.
The one is from the bottom left the other from the bottom right.

@Christopher2222 I agree, but that works (at least).

Notice that the Interrupt button in Maple 2025 is located at the bottom left of the screen (not in the file menu).
There are 2 buttons: Interrupt the current operation, Debug the current operation to the left of the text showing either Ready or Evaluating.
Unfortunately the Interrupt button doesn't work.
Try the uploaded worksheet below in Maple 2025, and in Maple 2024.
You cannot stop Maple 2025, but you can easily stop Maple 2024.

Interrupt_button_problem.mw 

Good question.

solve(x^n=0,x); # Zero
solve(x^n=0,x) assuming n<0;  # NULL
solve(x^n=0,x) assuming n>0;  # Zero
solve(x^n=0,x) assuming n=0;  # Null
0^0;  # 1
eval(x^n, {x=0,n=0}); # 1

Note:

0^(1/n);  # Returns unevaluated i.e. 0^(1/n)

@mmcdara  This also works in the very old Maple 12.

The problem is that it doesn't open in MaplePrimes, but in
https://bountygames.co.in/

The "post" is really just that link.

I have just tried to remove the post "Drive results with a top Google ads agency in the USA!"

It didn't work. It said "Error generating page".