Why does yum fail with error "[Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 403" ?

Issue

• Yum update with the following error:

# yum update
Loaded plugins: product-id, refresh-packagekit, rhnplugin, security, subscription-manager  
Updating certificate-based repositories.  
https://cdn.redhat.com/content/dist/rhel/server/6/6Server/x86_64/os/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 403"  
Trying other mirror.  
Setting up Update Process  
No Packages marked for Update  
# yum install gcc  
Loaded plugins: product-id, refresh-packagekit, rhnplugin, security, subscription-manager  
Updating certificate-based repositories.  
https://cdn.redhat.com/content/dist/rhel/server/6/6Server/x86_64/os/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 403"  
Trying other mirror.

• Yum commands fail with [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 403 Forbidden""
• System is registered to RHN classic and proxy is used in between. Facing the below-mentioned error while installing packages :

# yum update
Loaded plugins: product-id, rhnplugin, security
This system is receiving updates from RHN Classic or RHN Satellite.
Setting up Update Process
https://cdn.redhat.com/content/dist/rhel/server/6/6Server/x86_64/os/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 403"
Trying other mirror.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: rhel-6-server-rpms. Please verify its path and try again