*** Bug 1564329 has been marked as a duplicate of this bug. ***
Observed the same issue for rhel-7-server-rpms 7.4 release in a customer environment today.
We set the download policy to immediate which allowed the sync to begin without failing immediately. We are waiting to hear from the customer whether the sync was successful or if it failed at a later point.
Echoing wclark's comment here with my case as well (02082192). I have added this to the list of involved cases.