The backup is too outdated I'm afraid. The best chance I figured is to manually remove all the non-related mysql lines and import it again via myAdmin. (Which would be a huge task considering 280 000 + lines) So we really need to think if it is worth the trouble. 

Lesson well learned. Thanks for the reply!

Also all the files added to the group have been removed from the dataroot. So it wouldn't be enough to restore just the database.

Yes, you are right. But still got those! (Also once a month a complete ftp backup)

Normally you would run this on the backend via control panel from your hosting. Cron isn't an Elgg thing, it's a server thing.

You can disable the access level handling within the callback function you register for the cron plugin hook. Then you can access entities during the cronjob execution that would require an admin to be logged in otherwise.

Register in the init function of your plugin:

elgg_register_plugin_hook_handler('cron', 'daily', 'my_cronjob');

and then add the callback function to be executed during the daily cronjob:

function my_cronjob($event, $object_type, $object) {

    $access = elgg_set_ignore_access(true);
    $access_status = access_get_show_hidden_status();
    access_show_hidden_entities(true);

    <now the code that you want to be executed>

    access_show_hidden_entities($access_status);
    elgg_set_ignore_access($access);
}

Just exactly what I needed! Many thanks.