Thanks for information and thoughts!
I'm not intending to repair it, maybe it could be done, as Fabiofarelli found in FB. Nearby there are no experienced CV bike welders that I am aware of, and, in any case, I am (overly)cautious about these things.
I'm more curious about the causes even it will not be repaired, I like to find out about these things. Could lateral flex induce these, like a fatique crack from lateral motion?

I'm quite surprised it could be not that dangerous. When I took it for a spin I was thinking it could disintegrate any moment I will think a little bit more and maybe it will be my easy cruising bike someday.