Katello: new repository added to product, unable to fetch software from this repository -> "HTTPS Error 503 - Service Unavailable"

Problem: After adding a Repository to a product (product CentOS should also contain an EPEL repository) the client is unable to fetch from this repository.

Result on the client:
qpid-proton-c-0.24.0-1.el7.x86 FAILED
https://lxcfg1.uibk.ac.at/pulp/repos/ZID-Zentrale-Systeme/Library/CentOS_7_5_view/custom/CentOS_7_5_1804/EPEL_7_x86_64/Packages/q/qpid-proton-c-0.24.0-1.el7.x86_64.rpm: [Errno 14] HTTPS Error 503 - Service Unavailable

Apache error log foreman-ssl_error_ssl.log
[Thu Oct 18 16:56:18.625951 2018] [proxy:error] [pid 28213] AH00940: HTTP: disabled connection for (

Expected outcome: package retrieval should be possible

Foreman and Proxy versions: 1.17.1

are you able to use wget to confirm if the RPM is downloadable? It looks like the 503 may be coming from the upstream repo.