404 Error when setting Up Slack-Slash-Commands on GitLab 11.8.1-ee Self-Hosted

Summary

I'm currently getting 404-Errors when trying to Access the Generated URL for Gitlab's Slack-Integration via SlashCommands in Gitlab 11.8.1-ee (self hosted - not licensed).

Steps to reproduce

Setting up the Slack-SlashCommands-Integration like described in the Documentation. Try calling the Api from Slack via the Command /testproject help. Returns a 404. Manually calling the generated URL brings the same.

Here's the generated URL: https://[domain]/api/v4/projects/36/services/slack_slash_commands/trigger

What is the current bug behavior?

404-Error on accessing the generated Trigger-URL

What is the expected correct behavior?

Should work fine.

Relevant logs and/or screenshots

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Results of GitLab environment info

Expand for output related to GitLab environment info 
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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 >= 8.4.4 ? ... OK (8.4.4)
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: ... 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: ...
9/1 ... yes
6/2 ... yes
4/3 ... yes
4/4 ... yes
3/5 ... yes
3/6 ... yes
3/7 ... yes
4/8 ... yes
4/9 ... yes
4/10 ... yes
5/11 ... yes
9/12 ... yes
4/13 ... yes
9/14 ... yes
4/15 ... yes
4/16 ... yes
4/17 ... yes
7/18 ... yes
5/19 ... yes
4/20 ... yes
4/21 ... yes
5/22 ... yes
5/23 ... yes
4/24 ... yes
7/25 ... yes
5/26 ... yes
5/27 ... yes
5/28 ... yes
9/29 ... yes
9/30 ... yes
9/31 ... yes
8/32 ... yes
8/33 ... yes
8/34 ... yes
8/35 ... yes
5/36 ... yes
3/37 ... yes
8/38 ... yes
8/39 ... yes
8/40 ... yes
8/41 ... yes
8/42 ... yes
8/43 ... yes
8/44 ... yes
8/45 ... yes
8/46 ... yes
8/47 ... yes
8/48 ... yes
8/49 ... yes
8/50 ... yes
8/51 ... yes
8/52 ... yes
8/53 ... yes
8/54 ... yes
8/55 ... yes
9/58 ... yes
8/59 ... yes
8/60 ... yes
7/61 ... yes
4/62 ... yes
5/63 ... yes
7/65 ... yes
5/66 ... yes
9/67 ... yes
9/68 ... yes
9/69 ... yes
9/70 ... yes
4/71 ... yes
8/72 ... yes
8/74 ... yes
4/75 ... yes
4/76 ... yes
20/77 ... yes
9/78 ... yes
4/79 ... yes
9/80 ... yes
9/81 ... yes
9/82 ... yes
9/83 ... yes
8/84 ... yes
8/85 ... yes
8/86 ... yes
8/87 ... yes
8/88 ... yes
8/89 ... yes
7/90 ... yes
7/91 ... yes
4/94 ... yes
9/95 ... yes
8/96 ... yes
19/97 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.5 ? ... yes (2.5.3)
Git version >= 2.18.0 ? ... yes (2.18.1)
Git user has default SSH configuration? ... yes
Active users: ... 11
Elasticsearch version 5.6 - 6.x? ... skipped (elasticsearch is disabled)


Checking GitLab App ... Finished


Checking GitLab subtasks ... Finished