This is a repost from here:
https://forum.gitlab.com/t/deploying-to-package-registry-in-self-hosted-gitlab-instance/106268, sorry for that.

Essentially, my question is: how to get more information about the unexpected behaviour, that my self-hosted GitLab instance rejects any package deployments to the package registry with a

400 Bad Request

response.

That said, the request itself seems to be ok, as the same request (& pyirc configuration form) leads to success on https://test.pypi.org/legacy/.