Problem ocurred because my layout.tsx was in path folder that is in brackets (client) and I placed my error.tsx in the same folder expecting that error page would be shown inside layout.
If I properly understood, because my error.tsx was in path folder in (client), Nextjs. When I moved it one layout higher, it started working. And then i understood that in my situation error.tsx needs it's own layout not inherit it from (client) folder's layout, so i left error.tsx there.