After registering a DomU with RHN with valid entitlements and configuration I receive the error message: Insufficient Software Channel Entitlements (Error Class Code: 70)

Issue

• After registering Dom0 and a DomU in RHN with the procedure indicated in How do I register my Red Hat Enterprise Linux Virtual Guest system with Red Hat Network Classic? and having enough subscriptions, it's not possible to register another DomU.
• The following error appears:

Error Message: 
    Insufficient Software Channel Entitlements: 

    example  Red Hat Enterprise Linux (core server) 

note: One of the above Software Channel Entitlement(s) are required 
to provides access to: example Red Hat Enterprise Linux (v. 5 for 64-bit x86_64) 
Error Class Code: 70 
Error Class Info:  
     All available subscriptions for the requested channel have been exhausted. 
     Please contact a Red Hat Network Sales associate. 
Explanation:  
     An error has occurred while processing your request. If this problem 
     persists please enter a bug report at bugzilla.redhat.com. 
     If you choose to submit the bug report, please be sure to include 
     details of what you were trying to do when this error occurred and 
     details on how to reproduce this problem.