I have done a very careful handling and setup, nevertheless, there
definitely is a problem because the image I see in the eyepiece is
blocked (at the center of the image) by the shape (shadow) of the
internal diagonal mirror.
Am I missing something?
Is my telescope somehow broken?
Could you please shed some light on my issue?

You should only see this effect during the day. If you look at night, the
shadow of the secondary will disappear. To put you at ease: no, you haven't
done anything wrong, and this is not a defect of the scope.
--
Curtis Croulet
Temecula, California
33°27'59"N, 117°05'53"W