It is really odd that both servers have the same issue and I'm glad you've found a solution. The releasever is updated either by app-base or its dependency, clearos-release-community and I think it is read from /etc/clearos-release during the yum command. That is why I was heading to manually updating the rpms.

That probably would have worked. I figured out what was going on, though I don't know why. Rather than pulling $releasever=6, it was pulling $releasever=$releasever. So I ran echo "6" > /etc/yum/vars/releasever and then ran your previous recommendation of updating app-base, and that worked. Then I removed the faked in variable and it still worked as it had updated the release rpm while updating app-base.

This is getting trickier and I wonder if there is a config issue. Anyway, try this:

wget http://mirror.clearfoundation.com/clearos/community/6.7.0/updates/x86_64/RPMS/app-base-1.8.7-1.v6.noarch.rpm

wget http://mirror.clearfoundation.com/clearos/community/6.7.0/updates/x86_64/RPMS/app-base-core-1.8.7-1.v6.noarch.rpm

wget http://mirror.clearfoundation.com/clearos/community/6.7.0/updates/x86_64/RPMS/clearos-release-community-6-6.8.0.3.v6.noarch.rpm

wget http://mirror.clearfoundation.com/clearos/community/6.7.0/updates/x86_64/RPMS/cpupowerutils-1.3-1.el6.x86_64.rpm

wget http://mirror.clearfoundation.com/clearos/community/6.7.0/updates/x86_64/RPMS/python-urlgrabber-3.9.1-11.el6.noarch.rpm

wget http://mirror.clearfoundation.com/clearos/community/6.7.0/updates/x86_64/RPMS/yum-3.2.29-75.el6.centos.noarch.rpm

rpm -Uvh *.rpm

yum clean all && yum update

Grrr...

Could not retrieve mirrorlist http://mirrorlist.clearfoundation.com/clearos/mirrorlist/clearos-community-updates-$releasever error was
14: PYCURL ERROR 22 - "The requested URL returned error: 404 Not Found"
Error: Cannot find a valid baseurl for repo: clearos-updates

Happening on 2 different servers at 2 different companies

OK. Try:

yum update app-base --disablerepo=* --enablerepo=clearos-updates

I tried that and unfortunately get this error:

Could not retrieve mirrorlist http://mirrorlist.clearfoundation.com/clearos/mirrorlist/clearos-community-os-$releasever error was
14: PYCURL ERROR 22 - "The requested URL returned error: 404 Not Found"
Error: Cannot find a valid baseurl for repo: clearos

Link with answer