"The redirect URI included is not valid" error when setting up GItLab Pages without wildcards

Summary

When setting up GitLab Pages manually without a wildcard DNS record, we always receive a The redirect URI included is not valid error during authentication for GitLab Pages behind access restrictions.

Steps to reproduce

We have a self-hosted GitLab instance and would like to add Pages support. However, for our organization it is not possible to get wildcard DNS entries. Since we only need to support a (very) finite list of projects for Pages, we wanted to add DNS A records for Pages manually. This is what we tried:

• GitLab instance is at gitlab.subdomain.example.com
• Pages are to be available on the same server under pages.subdomain.example.com, which is a CNAME to gitlab.subdomain.example.com
• Additionally, we have also have A records for projects.pages.subdomain.example.com and myproject.pages.subdomain.example.com pointing to the same server.

All Pages URLs are HTTPS-only and the GitLab Pages DNS certificate was issued for all three pertinent domains, i.e., pages..., projects.pages..., and myproject.pages...

Accessing myproject.pages.subdomain.example.com seems to work at first, and since it is configured to be only available for authenticated users, it redirects to an authentication page. However, then we get shown a

An error has occurred

The redirect URI included is not valid.

error and from there it is not possible (at least for me) to find out what went wrong and where. When we try to test the setup with a different domain for GitLab Pages for which we can create wildcard DNS A records, everything works as advertised. However, I still feel like this is unexpected behavior and thus I am filing a bug report.

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:		Ubuntu 18.04
Current User:	git
Using RVM:	no
Ruby Version:	2.6.5p114
Gem Version:	2.7.10
Bundler Version:1.17.3
Rake Version:	12.3.3
Redis Version:	5.0.7
Git Version:	2.26.2
Sidekiq Version:5.2.7
Go Version:	unknown

GitLab information
Version:	12.10.2
Revision:	d90add1c62d
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	11.7
URL:		https://gitlab.subdomain.example.com
HTTP Clone URL:	https://gitlab.subdomain.example.com/some-group/some-project.git
SSH Clone URL:	git@gitlab.subdomain.example.com:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers: 

GitLab Shell
Version:	12.2.0
Repository storage paths:
- default: 	/mnt/ssd/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 >= 12.2.0 ? ... OK (12.2.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: 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: ... LDAP is disabled in config/gitlab.yml
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: ...
3/1 ... yes
18/2 ... yes
19/3 ... yes
32/4 ... yes
2/5 ... yes
34/13 ... yes
14/16 ... yes
14/17 ... yes
14/18 ... yes
14/19 ... yes
14/20 ... yes
14/21 ... yes
14/22 ... yes
14/23 ... yes
14/24 ... yes
14/25 ... yes
14/26 ... yes
14/27 ... yes
19/28 ... yes
19/29 ... yes
19/30 ... yes
19/31 ... yes
19/32 ... yes
19/33 ... yes
19/34 ... yes
19/35 ... yes
19/36 ... yes
19/37 ... yes
19/38 ... yes
19/39 ... yes
19/40 ... yes
19/41 ... yes
23/43 ... yes
23/44 ... yes
22/45 ... yes
15/46 ... yes
6/48 ... yes
6/49 ... yes
19/50 ... yes
51/51 ... yes
51/52 ... yes
51/53 ... yes
50/54 ... yes
24/55 ... yes
24/56 ... yes
7/57 ... yes
49/58 ... yes
50/59 ... yes
52/61 ... yes
2/62 ... yes
51/64 ... yes
47/65 ... yes
47/66 ... yes
21/67 ... yes
31/68 ... yes
21/69 ... yes
55/70 ... yes
21/71 ... yes
21/72 ... yes
21/73 ... yes
31/74 ... yes
14/75 ... yes
34/76 ... yes
6/77 ... yes
6/78 ... yes
6/79 ... yes
25/80 ... yes
21/81 ... yes
26/82 ... yes
26/83 ... yes
17/84 ... yes
6/85 ... yes
23/86 ... yes
23/87 ... yes
18/89 ... yes
25/90 ... yes
21/91 ... yes
22/92 ... yes
21/93 ... yes
60/94 ... yes
60/95 ... yes
61/96 ... yes
23/97 ... yes
26/98 ... yes
21/100 ... yes
23/102 ... yes
26/104 ... yes
65/106 ... yes
64/107 ... yes
64/108 ... yes
65/109 ... yes
19/110 ... yes
76/111 ... yes
21/112 ... yes
25/113 ... yes
47/114 ... yes
17/115 ... yes
17/116 ... yes
17/117 ... yes
69/118 ... yes
68/119 ... yes
34/121 ... yes
68/122 ... yes
26/123 ... yes
34/124 ... yes
21/125 ... yes
26/126 ... yes
25/127 ... yes
34/128 ... yes
34/129 ... yes
29/130 ... yes
22/131 ... yes
69/132 ... yes
76/134 ... yes
76/135 ... yes
23/137 ... yes
23/138 ... yes
23/139 ... yes
23/142 ... yes
Redis version >= 4.0.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.5)
Git version >= 2.22.0 ? ... yes (2.26.2)
Git user has default SSH configuration? ... yes
Active users: ... 32
Is authorized keys file accessible? ... yes
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished