The above error is caused by a known bug in version 0.5.3-6.el5_2.6 of the package yum-rhn-plugin. The bug is triggered when third party (non-RHN) repositories are configured with Yum.
The recommended workaround is to downgrade the yum-rhn-plugin package to a previous version. To do this, download the old version of the package from the Internet, and then run:
# rpm -Uvh --oldpackage yum-rhn-plugin-0.5.2-3.el5.noarch.rpm