Alertmanager down

Summary

I have Gitlab installed for a few months now. From the first installation on, the status of the "alertmanager" has been down. I suppose this is also why I don't get any email notifications about repo changes or if I send an email to all users via the admin panel?

$ sudo gitlab-ctl status
>>> 
down: alertmanager: 619129s, normally up; run: log: (pid 29105) 3290880s
run: gitaly: (pid 6141) 619038s; run: log: (pid 29111) 3290879s
run: gitlab-monitor: (pid 6179) 619036s; run: log: (pid 29099) 3290880s
run: gitlab-workhorse: (pid 6125) 619039s; run: log: (pid 29090) 3290880s
run: logrotate: (pid 12641) 3423s; run: log: (pid 29094) 3290880s
run: nginx: (pid 6190) 619036s; run: log: (pid 29107) 3290879s
run: node-exporter: (pid 6198) 619035s; run: log: (pid 29113) 3290879s
run: postgres-exporter: (pid 6205) 619035s; run: log: (pid 29110) 3290879s
run: postgresql: (pid 29117) 3290879s; run: log: (pid 29116) 3290879s
run: prometheus: (pid 6289) 619034s; run: log: (pid 29092) 3290880s
run: redis: (pid 29097) 3290880s; run: log: (pid 29096) 3290880s
run: redis-exporter: (pid 6303) 619034s; run: log: (pid 29118) 3290879s
run: sidekiq: (pid 9925) 4465s; run: log: (pid 29120) 3290879s
run: unicorn: (pid 6317) 619033s; run: log: (pid 29088) 3290880s

Relevant logs and/or screenshots

The file /var/log/gitlab/alertmanager/current is empty

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:         Ubuntu 18.04
Proxy:          no
Current User:   git
Using RVM:      no
Ruby Version:   2.4.4p296
Gem Version:    2.7.6
Bundler Version:1.16.2
Rake Version:   12.3.1
Redis Version:  3.2.11
Git Version:    2.18.0
Sidekiq Version:5.1.3
Go Version:     unknown
GitLab information
Version:        11.2.3-ee
Revision:       aadca994104
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
DB Version:     9.6.8
URL:            http://$VERY_SECRET_IP
HTTP Clone URL: http://$VERY_SECRET_IP/some-group/some-project.git
SSH Clone URL:  git@46.16.74.149:some-group/some-project.git
Elasticsearch:  no
Geo:            no
Using LDAP:     no
Using Omniauth: no
GitLab Shell
Version:        8.1.1
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

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 Sidekiq ...
Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Reply by email is disabled in config/gitlab.yml
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: ...
3/2 ... yes
3/3 ... yes
3/4 ... yes
3/8 ... yes
6/9 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.5 ? ... yes (2.4.4)
Git version >= 2.9.5 ? ... yes (2.18.0)
Git user has default SSH configuration? ... yes
Active users: ... 7
Elasticsearch version 5.1 - 5.5? ... skipped (elasticsearch is disabled)
Checking GitLab ... Finished