Can't add new SSH keys to user profile ("ADD KEYS" button grayed out)

Summary

When attempting to add a new SSH key on my Gitlab server (https://gitlab-url.bla/profile/keys/new) the "ADD KEY" button remains grayed out and I am unable to add a new key (removing keys works fine). I have tried to add RSA, DSA and ED25519 keys, none of them work. This issue affects all users on the server.

Steps to reproduce

Vistit https://gitlab-url.bla/profile/keys/new
Paste public key into "Key" dialogue
Press tab and allow the "Title" dialogue to auto-populate
Attempt to press the "ADD KEY" button

Expected behavior

Normally, the "ADD KEY" button would be enabled once the "Key" and "Title" dialogues are populated at which point you can simply press and the key will be added to the user profile.

Relevant logs and/or screenshots

N/A

Output of checks

Results of GitLab Application Check

Checking GitLab Shell ...

GitLab Shell version >= 2.6.9 ? ... OK (2.6.9)
Repo base directory exists? ... yes
Repo base directory is a symlink? ... no
Repo base owned by git:git? ... yes
Repo base access is drwxrws---? ... yes
hooks directories in repos are links: ... 
3/2 ... ok
3/3 ... ok
3/4 ... ok
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Check directories and files: 
	/var/opt/gitlab/git-data/repositories: OK
	/var/opt/gitlab/.ssh/authorized_keys: OK
Test redis-cli executable: redis-cli 2.8.21
Send ping to redis server: PONG
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 with autocrlf=input? ... yes
Database config exists? ... yes
Database is SQLite ... no
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config outdated? ... no
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory setup correctly? ... 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: ... 
3/2 ... yes
3/3 ... yes
3/4 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.1.0 ? ... yes (2.1.7)
Your git bin path is "/opt/gitlab/embedded/bin/git"
Git version >= 1.7.10 ? ... yes (2.6.1)
Active users: 5

Checking GitLab ... Finished

Results of GitLab Environment Info

System information
System:		Ubuntu 12.04
Current User:	git
Using RVM:	no
Ruby Version:	2.1.7p400
Gem Version:	2.5.1
Bundler Version:1.10.6
Rake Version:	10.4.2
Sidekiq Version:4.0.1

GitLab information
Version:	8.3.2
Revision:	fbb8b6e
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		https://XXX.XXX.XXX.XXX
HTTP Clone URL:	https://XXX.XXX.XXX.XXX/some-group/some-project.git
SSH Clone URL:	git@XXX.XXX.XXX.XXX:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	no

GitLab Shell
Version:	2.6.9
Repositories:	/var/opt/gitlab/git-data/repositories
Hooks:		/opt/gitlab/embedded/service/gitlab-shell/hooks/
Git:		/opt/gitlab/embedded/bin/git