The problem appears on more sites, unfortunately…
1. When i click on the post title, and gets redirected to the certain post site (f.e. – http://www.example.pl/2018/09/07/My-trip-from-nowhere.html. ). Still not custom header (no idea where to define it tho).
2. When i enter “category” (of posts) site – the place where i see all posts from certain category. Same issue = default header instead of custom.