The same problem for all our cPanel/WHM - (11.38.1 (build 13)) servers, all munin installs is manual, not as cpanel plugin.
[email protected] [~]# rpm -q munin
munin-2.0.16-1.el6.noarch
Every day i get "Cpanel update failure during updatenow".
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[20130717.041905] Testing RPM transaction...