subscription-manager register fails with 'NoneType' exception when using new account

I want to change my subscription form a private developer account to a new company. Registering with the new account fails, while it does work with the old account.

root@kal4:~# subscription-manager unregister
Unregistering von: subscription.rhsm.redhat.com:443/subscription

root@kal4:~# subscription-manager register --username=newuser --password=newpw

Registrieren bei: subscription.rhsm.redhat.com:443/subscription
'NoneType' object has no attribute 'getitem'

root@kal4:~# subscription-manager register --username=olduser --password=oldpw
Registrieren bei: subscription.rhsm.redhat.com:443/subscription
Das System wurde mit ID registriert: 8e236287-0bf6-4ec5-8642-10cdb4682571
Der registrierte Systemname lautet: kal4

System:
Red Hat Enterprise Linux Server release 7.8 (Maipo)

• regards, R2H2