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
6/7 … yes
4/9 … yes
2/12 … yes
2/13 … yes
2/14 … yes
10/15 … yes
5/18 … yes
8/20 … yes
8/22 … yes
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2/25 … yes
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8/34 … yes
8/35 … yes
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8/40 … yes
8/41 … yes
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2/53 … yes
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2/55 … yes
8/59 … yes
6/60 … yes
2/62 … yes
8/63 … yes
3/65 … yes
7/66 … yes
2/67 … yes
4/69 … yes
5/70 … yes
4/72 … yes
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6/74 … yes
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6/76 … yes
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8/80 … yes
8/81 … yes
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5/84 … yes
30/85 … yes
2/87 … yes
8/88 … yes
11/91 … yes
11/92 … yes
4/93 … yes
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30/95 … yes
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32/100 … yes
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30/114 … yes
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2/117 … yes
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3/121 … yes
28/124 … yes
3/125 … yes
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3/127 … yes
8/128 … yes
30/129 … yes
33/130 … yes
4/131 … yes
40/132 … yes
11/133 … yes
11/134 … yes
6/135 … yes
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3/137 … yes
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40/139 … yes
8/140 … yes
12/141 … yes
4/142 … yes
3/143 … yes
33/145 … yes
7/147 … yes
6/148 … yes
12/149 … yes
59/150 … yes
50/151 … yes
33/152 … yes
9/155 … yes
3/156 … yes
28/157 … yes
3/158 … yes
12/159 … yes
12/160 … yes
63/161 … yes
8/162 … yes
52/163 … yes
62/164 … yes
11/165 … yes
4/166 … yes
64/167 … yes
62/168 … yes
40/169 … yes
62/170 … yes
6/171 … yes
7/172 … yes
50/173 … yes
8/174 … yes
12/175 … yes
64/176 … yes
50/177 … yes
8/178 … yes
7/179 … yes
28/180 … yes
65/181 … yes
8/182 … yes
68/183 … yes
7/184 … yes
62/185 … yes
7/186 … yes
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