If you marked Publish via HTTP for the Repository in Katello, you can open the URL in your Browser by swapping out katello:// with http:// AND appending a / (the latter is important for pulp to work correctly).
E.g. http://foreman-z01.infra.zgt.local/pulp/deb/ICT-ZGT/acceptatie/CV_Ubuntu-18/custom/Ubuntu/Ubuntu_Subscription-manager/
This way you can see more closer, if there is something wrong with the repository provided by pulp.
The 404 suggests that e.g. the Release file(s) are not there.
The latter can be fixed by Regenerate Metadata for the respective Content-View Versions.