GitLab Redirect URI for project import invalid

I created my own GitLab CE Instance and want to setup the importer for my projects from GitLab.com

I followed all steps from Integrate your server with GitLab.com | GitLab (which are quite unclear in my opinion) but when I click on “GitLab.com” in the Import Project View I get redirected to GitLab.com with the error “The redirect URI included is not valid.”
I tried various solutions from google (like adding a / at the end) but none of them worked.

http://code.xxxx.yyy/import/gitlab/callback
http://code.xxxx.yyy/users/auth/gitlab/callback

These are my callback URLs (with domain censored). Using https:// didn’t work as well.

The same error occurs when trying to sign up with gitlab.