As you found out that mirror is having problems and is not in sync at the moment ( http://mirror-status.nethserver.org/ ).
You can remove /var/cache/yum/x86_64/7/timedhosts.txt for fastestmirror yum plugin to check speed of mirrors and regenerate the file. In case it does not work you can exclude a specific mirror adding it to /etc/yum/pluginconf.d/fastestmirror.conf (exclude=ns-mirror3.ibi.net.br).
Or if it is a temporary measure, you disable fastestmirror plugin at runtime with:
yum --disableplugin=fastestmirror update
Is the server behind a proxy?
You can try with:
yum --setopt=http_caching=none update