Yum failed with 'Cannot retrieve repository metadata (repomd.xml) for repository: epel.'

Issue

• Running yum update gives the below error:

Error communicating with server. The message was:
timed out
Setting up Local Package Process
Examining perl-Log-Log4perl-1.30-1.el6.noarch.rpm: perl-Log-Log4perl-1.30-1.el6.noarch
Marking perl-Log-Log4perl-1.30-1.el6.noarch.rpm to be installed
Error: Cannot retrieve repository metadata (repomd.xml) for repository: epel. Please verify its path and try again

[Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404"

• Repomd.xml file can be retrieved with wget or curl via http proxy, but it still fails to be retrieved by yum.