The system @maximilian used for opensource.com is still using Pulp 2, while Katello 4.0 uses exclusively Pulp 3. The Katello 4.0.1 release will include a newer version of pulp_deb for Pulp 3 with several critical bugfixes. My best recommendation for now is to wait for Katello 4.0.1 and then try again. If you are still having issues then, please open a new thread posting your exact error.
