Unable to activate Mattermost: HTTP 502 on mattermost_external_url

I am trying to activate mattermost on my gitlab server following the official docs.

following is my mattermost-configuration from /etc/gitlab/gitlab.rb:

mattermost_external_url 'http://192.168.178.130:7000'
mattermost['gitlab_enable'] = true
mattermost['gitlab_secret'] = "the_secret"
mattermost['gitlab_id'] = "the_id"
mattermost['gitlab_scope'] = ""
mattermost['gitlab_auth_endpoint'] = "http://192.168.178.130/oauth/authorize"
mattermost['gitlab_token_endpoint'] = "http://192.168.178.130/oauth/token"
mattermost['gitlab_user_api_endpoint'] = "http://192.168.178.130/api/v3/user"
mattermost['service_enable_incoming_webhooks'] = true
mattermost['email_enable_sign_up_with_email'] = false

I executed gitlab-ctl reconfigure without errors. GitLab works fine, but when I try to access mattermost on http://192.168.178.130:7000 I get a 502 Bad Gateway:

I should be worth to point out that /var/log/gitlab/mattermost/current gets spammed with

panic: Failed to create index pq: column "inviteid" does not exist

every second.

Gitlab is up to date. gitlab-rake gitlab:env:info outputs:

System information
System: Debian 8.4
Current User: git
Using RVM: no
Ruby Version: 2.1.8p440
Gem Version: 2.5.1
Bundler Version:1.10.6
Rake Version: 10.5.0
Sidekiq Version:4.1.2

GitLab information
Version: 8.8.2
Revision: 93450d3
Directory: /opt/gitlab/embedded/service/gitlab-rails
DB Adapter: postgresql
URL: http://192.168.178.130
HTTP Clone URL: http://192.168.178.130/some-group/some-project.git SSH Clone URL: git@192.168.178.130:some-group/some-project.git
Using LDAP: no
Using Omniauth: no

GitLab Shell
Version: 2.7.2
Repositories: /var/opt/gitlab/git-data/repositories
Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks/
Git: /opt/gitlab/embedded/bin/git