@Girays issue was our trophies tab, which called \IPS\Module::get('trophies','modulename') which works for everybody..
BUT, your plugin is calling the tab extensions (which are meant to be called only from the frontend), from the ACP, so the module can't be found.
I have adjusted now my code and passed the 3rd parameter for the "area" to search for a frontend module.
/**
* Get a module
*
* @param string $app
* @param string $key
* @param string|NULL $area
* @return \IPS\Application\Module
* @throws \OutOfRangeException
*/
public static function get( $app, $key, $area=NULL )
I guess this could be useful for other coders too..