Why registration fails with an error "RestlibException: Problem creating unit Consumer" ?

Issue

• Registration fails with an error RestlibException: Problem creating unit Consumer

Problem creating unit Consumer [id=8a85f98a4a6df197014aa9e59b1234a, type = ConsumerType [id=0, label=system], getName() = hostname.example.org]

• Registration fails:

[root@rhel7a ~]# subscription-manager register --username=rhn-user 
パスワード: 
ユニット Consumer [id = 8a85f9834d8510ee014d94c4750c5e37, type = ConsumerType [id=0, label=system], getName() = hostname] の作成に問題があります。
[root@rhel7a ~]#