Yes, content of post-commit was the problem, cause it has been launched under user apache:apache which has /sbin/nologin option in /etc/passwd, at least on my system. So no one ever accessed /usr/bin/svn update and enetered user and pass for svn repo using user apache. In this case this helps:

chown -Rv apache.apache /usr/local/subversion/ /usr/local/apache2/

and in post-commit you type:

/usr/bin/svn update /usr/local/apache2/htdocs/ --username svn_user --password svn_pwd --non-interactive >> /usr/local/subversion/repository/logs/post-commit.log

 Cheers!