My install of CodeMirror is also returning the following error
[2016-11-03 14:50:30] (WARN @ /home/holidayproperty/public_html/new/core/xpdo/xpdo.class.php : 542) Could not load package metadata for package codemirror.
A good host can disable just the rule(s) that cause trouble, or they can disable mod_security for just the Manager directory where they're not really necessary, assuming that you trust the people you let into the Manager.