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:
The effect of this incompatibility is subtle: Numbered Headings works fine just as before, but now if you try to insert any macro that has a body using '+' icon, you get an endless spinner, and a Javascript 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.