'yum --security check-update' reports some errors.

Issue

• The host was registered by RHN classic and subscription-manager, then after unregistration of subscription-manager, the following messages were output by executing yum.

Downloading Packages:
https://cdn.redhat.com/content/dist/rhel/server/6/6Server/x86_64/os/Packages/libxml2-2.7.6-14.el6_5.1.x86_64.rpm:
[Errno 14] PYCURL ERROR 22 - "NSS: client certificate not found: /etc/pki/entitlement/8016347195082143299.pem"
Trying other mirror.
https://cdn.redhat.com/content/dist/rhel/server/6/6Server/x86_64/os/Packages/libxml2-python-2.7.6-14.el6_5.1.x86_64.
rpm: [Errno 14] PYCURL ERROR 22 - "NSS: client certificate not found: /etc/pki/entitlement/8016347195082143299.pem"
Trying other mirror.

• The customer wants to know why the message appears and workaround.