There is some discussion going on in the related Jira: https://issues.redhat.com/browse/SAT-35513
Debugging is happening but it’s a bit slow due to the complexity of the issue.
I think we may need to revert the version of createrepo_c in the meantime. Since Katello 4.18 GA is supposed to be today, there should be news soon.
Edit: we are indeed delaying Katello 4.18.0 GA for this issue.