Hi @derozone ,
vSphere replication registration to vCenter requires SSO administrator credentials. This is required to access to SSO service registrations on vCenter and create solution users vSphere replication.
User needs to be the one with administrator privileges to vCenter Single Sign-On, so you can try adding a user to SSO administrator groups and trying to register it.
Add vCenter Single Sign-On Users Add Members to a vCenter Single Sign-On Group
Hope that helps