Ok, thank you everyone for comments and suggestions. I initially suspected multiple reflections due to something wrong inside the lens. Now, I'm almost sure in the root cause.
There is glass separation in the secondary mirror. The mirror itself consists of mirror + glued lens(es), and now they are partially separated (see photo). Due to asymmetrical separation, resulting picture is also asymmetrical. Multiple reflections on the glass-air interface are producing that pattern.
I can try to shadow "bad" part, leaving only central part of the mirror to work, but in this case lens will become F/16
, or something like.