After turning on the Mattermost service is available only logo

1) I just uncomment the line with external_url

#####################
# GitLab Mattermost #
#####################

mattermost_external_url 'http://chat.mydomain.com'
#
# mattermost['enable'] = false
# mattermost['username'] = 'mattermost'
# mattermost['group'] = 'mattermost'
# mattermost['home'] = '/var/opt/gitlab/mattermost'
# mattermost['database_name'] = 'mattermost_production'

All other lines are commented

2) The entire screenshot with incognito window

Screen Shot 2015-12-14 at 19.41.25.png1433x875 52.8 KB