Edit file /home/gitlab/gitlab/config/unicorn.rb
Find line listen "#{app_dir}/tmp/sockets/gitlab.socket" and comment it.
Uncomment line listen "127.0.0.1:8080"
sudo a2enmod proxysudo a2enmod proxy_balancersudo a2enmod proxy_httpsudo a2enmod rewrite
Thanks a lot. This is my gitlab.conf that did the trick (note the different version numbers in /gems/ lines):