CSS files 404 after updating GitLab CE to 9.3.4+

Summary

After I updated from 9.3.3 to 9.3.4 the webinterface broke. No CSS files are being loaded and a quick check revealed that they are 404ing. The problem persits up till 9.3.6.

Steps to reproduce

Install GitLab CE 9.3.4+

What is the current bug behavior?

All requests for CSS files return 404.

What is the expected correct behavior?

The CSS files loading normally.

Relevant logs and/or screenshots

https://pageshot.net/WIZVUIO7y3gor0ZO/gitlab.brainstonemod.com

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:         Debian 8
Current User:   git
Using RVM:      no
Ruby Version:   2.3.3p222
Gem Version:    2.6.6
Bundler Version:1.13.7
Rake Version:   10.5.0
Redis Version:  3.2.5
Git Version:    2.13.0
Sidekiq Version:5.0.0
Go Version:     unknown
GitLab information
Version:        9.3.4
Revision:       3888c01
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
URL:            https://gitlab.brainstonemod.com
HTTP Clone URL: https://gitlab.brainstonemod.com/some-group/some-project.git
SSH Clone URL:  git@gitlab.brainstonemod.com:some-group/some-project.git
Using LDAP:     no
Using Omniauth: yes
Omniauth Providers: github
GitLab Shell
Version:        5.1.1
Repository storage paths:

• default: /var/opt/gitlab/git-data/repositories Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab Shell ...
GitLab Shell version >= 5.1.1 ? ... OK (5.1.1)
Repo base directory exists?
default... yes
Repo storage directories are symlinks?
default... no
Repo paths owned by git:root, or git:git?
default... yes
Repo paths access is drwxrws---?
default... yes
hooks directories in repos are links: ...
3/5 ... ok
3/9 ... ok
10/13 ... ok
10/14 ... ok
10/15 ... ok
3/19 ... ok
13/20 ... ok
13/21 ... ok
13/22 ... ok
13/23 ... ok
13/24 ... ok
13/25 ... ok
13/26 ... ok
13/27 ... ok
13/28 ... ok
13/29 ... ok
13/30 ... ok
13/31 ... ok
13/32 ... ok
13/33 ... ok
13/34 ... ok
13/35 ... ok
3/36 ... ok
3/37 ... ok
13/38 ... ok
3/42 ... ok
16/43 ... ok
16/44 ... ok
13/45 ... ok
16/46 ... ok
16/47 ... ok
16/48 ... ok
16/49 ... ok
16/50 ... ok
16/51 ... ok
16/52 ... ok
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
Send ping to redis server: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Sidekiq ...
Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Reply by email ...
Reply by email is disabled in config/gitlab.yml
Checking Reply by email ... Finished
Checking LDAP ...
LDAP is disabled in config/gitlab.yml
Checking LDAP ... Finished
Checking GitLab ...
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: ...
3/5 ... yes
3/9 ... yes
10/13 ... yes
10/14 ... yes
10/15 ... yes
3/19 ... yes
13/20 ... yes
13/21 ... yes
13/22 ... yes
13/23 ... yes
13/24 ... yes
13/25 ... yes
13/26 ... yes
13/27 ... yes
13/28 ... yes
13/29 ... yes
13/30 ... yes
13/31 ... yes
13/32 ... yes
13/33 ... yes
13/34 ... yes
13/35 ... yes
3/36 ... yes
3/37 ... yes
13/38 ... yes
3/42 ... yes
16/43 ... yes
16/44 ... yes
13/45 ... yes
16/46 ... yes
16/47 ... yes
16/48 ... yes
16/49 ... yes
16/50 ... yes
16/51 ... yes
16/52 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.3 ? ... yes (2.3.3)
Git version >= 2.7.3 ? ... yes (2.13.0)
Active users: ... 12
Checking GitLab ... Finished

Possible fixes

Downgrading to 9.3.3.