The problem with warnings and "blank page" seems to be solved, but plugin doesn't block users after reaching limits now. I chosen one user and defined 3 posts limit in any type (by USER_ID). I saved limit.
After relogin as my test user I can add as many posts as I want. My user is Contributor.
Plugin doesn't work even if I set a limit on a Role. I couldn't find any combination to make it works.