Maybe someone with a Newtonian telescope experienced the same problem as I did. After cleaning the primary mirror of my newt, BTW here is a nice tutorial on how to do it, I noticed that the brighter stars have a triangular shape. After some googling, I found the indications that I have pinched the primary mirror. Fortunately, it was just elastically deformed by the rubber clamps. When I placed the mirror back I overtightened the screws, therefore the mirror was not reflecting brighter stars properly.

By loosening the screws everything went back to normal, so the solution is quite simple.