Today the problem has magically gone away. No obvious explanation. I logged in, tried it out and it worked fine.

The code is simply this.

> restart;
> with(numapprox);
> coslower := proc (x, n) options operator, arrow; sum((-1)^i*x^(2*i)/factorial(2*i), i = 0 .. 2*n+1) end proc;
> cosupper := proc (x, n) options operator, arrow; sum((-1)^i*x^(2*i)/factorial(2*i), i = 0 .. 2*n+2) end proc;
> plot([cos(x), coslower(x, 0)], x = 0 .. 2);