Bad request when integrating with an on-premise Sentry installation

Summary

Integration with Sentry does not work on my Ominbus installation. Valid API address and valid token yield Connection has failed. Re-check Auth Token and try again error message.

Steps to reproduce

1. Create a token in Sentry
2. Go to GitLab project settings, provide the required credentials
3. Press Connect

What is the current bug behavior?

Connection has failed. Re-check Auth Token and try again error is shown

What is the expected correct behavior?

I should be able to select Sentry project which should be integrated

Relevant logs and/or screenshots

==> /var/log/gitlab/gitlab-rails/production.log <==
Processing by Projects::ErrorTrackingController#list_projects as JSON
  Parameters: {"error_tracking_setting"=>{"api_host"=>"https://sentry.applover.pl", "token"=>"[FILTERED]"}, "namespace_id"=>"theticketingco", "project_id"=>"theticketingco_backend", "error_tracking"=>{"error_tracking_setting"=>{"api_host"=>"https://sentry.applover.pl", "token"=>"[FILTERED]"}}}
Completed 400 Bad Request in 23ms (Views: 0.3ms | ActiveRecord: 2.2ms)
Started GET "/api/v4/projects/280" for 34.201.221.233 at 2019-09-19 08:11:27 +0000
Started GET "/api/v4/projects/280/issues?order_by=updated_at&page=1&per_page=100&sort=desc&state=all" for 34.201.221.233 at 2019-09-19 08:11:28 +0000

Implementation Plan

Quick win

1. Add #32314 (comment 341289508) to the troubleshooting section https://docs.gitlab.com/ee/operations/sentry_error_tracking.html#troubleshooting

Longer term

Improve the error messages???