OAuth sign-up not working

Hi, I am sorry if this is just a configuration problem, but I am unable to set up OmniAuth sign-up:

Summary

Users are unable to sign-up via github: You need to sign in or sign up before continuing.

Steps to reproduce

Follow https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/integration/omniauth.md :

gitlab_rails['omniauth_enabled'] = true
gitlab_rails['omniauth_allow_single_sign_on'] = ['github']
gitlab_rails['omniauth_block_auto_created_users'] = true
gitlab_rails['omniauth_auto_link_ldap_user'] = false

gitlab_rails['omniauth_providers'] = [
  {
    "name" => "github",
    "app_id" => "...",
    "app_secret" => "...",
    "args" => { "scope" => "user:email" }
  }
]

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:         Ubuntu 14.04
Current User:   git
Using RVM:      no
Ruby Version:   2.5.3p105
Gem Version:    2.7.6
Bundler Version:1.17.3
Rake Version:   12.3.2
Redis Version:  3.2.12
Git Version:    2.18.1
Sidekiq Version:5.2.5
Go Version:     unknown
GitLab information
Version:        11.10.4
Revision:       62c464651d2
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     PostgreSQL
DB Version:     9.6.11
URL:            https:///gitlab
HTTP Clone URL: https:///gitlab/some-group/some-project.git
SSH Clone URL:  git@:some-group/some-project.git
Using LDAP:     yes
Using Omniauth: yes
Omniauth Providers: github
GitLab Shell
Version:        9.0.0
Repository storage paths:

• default: /var/opt/gitlab/git-data/repositories GitLab Shell path: /opt/gitlab/embedded/service/gitlab-shell Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 9.0.0 ? ... OK (9.0.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Redis available via internal API: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
Gitaly: ... default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Incoming Email ...
Incoming Email: ... Reply by email is disabled in config/gitlab.yml
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... Server: ldapmain
LDAP authentication... Success
LDAP users with access to your GitLab server (only showing the first 100 results)
[removed for privacy...]
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
4/6 ... yes
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62/185 ... yes
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66/187 ... yes
65/188 ... yes
71/189 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.5.3)
Git version >= 2.18.0 ? ... yes (2.18.1)
Git user has default SSH configuration? ... yes
Active users: ... 69
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished