Discord Integration - Test fails

Whenever I try to add the discord integration, GitLab throws and error saying "Test failed. false". I can paste the webhook into my browser and I'm instantly faced with a json response.

I tried doing the same from my GitLab server by using curl and it was fully able to process the request.

The gitlab response however, looks like this:

{
  error: true
  message: "Test failed."
  service_response: "false"
  test_failed: true
}

Looking into the production.log I can see this:

1. Started PUT "/sem03/semesterprojekt/-/services/discord/test" for X.X.X.X at 2019-09-10 20:35:25 +0200

2. Processing by Projects::ServicesController#test as JSON

3. Parameters: {"utf8"=>"✓", "authenticity_token"=>"[FILTERED]", "service"=>{"active"=>"1", "push_events"=>"1", "issues_events"=>"1", "confidential_issues_events"=>"1", "merge_requests_events"=>"1", "note_events"=>"1", "confidential_note_events"=>"1", "tag_push_events"=>"1", "pipeline_events"=>"1", "wiki_page_events"=>"0", "webhook"=>"[FILTERED]", "notify_only_broken_pipelines"=>"1", "notify_only_default_branch"=>"1"}, "namespace_id"=>"sem03", "project_id"=>"semesterprojekt", "id"=>"discord"}

4. Completed 200 OK in 116ms (Views: 0.3ms | ActiveRecord: 24.1ms)

I'm not entirely sure where I'm going wrong :/