The Confluence Numbered Headings plugin is one of those plugins that was free for many years, then went commercial.
Until recently it still worked, but not any more. As of Confluence 7.1 and later the UPM flags the old version as incompatible:
AJS.clone is not a function error:
The good news is that after upgrading, the plugin is still free: it just as a 'Pro' version:
So upgrade away, and if your budget permits, support the plugin authors by paying for the Pro version.