function discussion_get_subscriptions($hook, $type, $subscriptions, $params) {

$reply = $params['event']->getObject();


if (!elgg_instanceof($reply, 'object', 'discussion_reply')) {

return $subscriptions;

}


$container_guid = $reply->getContainerEntity()->container_guid;

$container_subscriptions = elgg_get_subscriptions_for_container($container_guid);


return ($subscriptions + $container_subscriptions);

}

It might be another plugin causing the problem. It might occur only in some groups if this other plugin provides some functionality that's not in use in all groups or it allows adding some content that is not present in groups without the issue. For testing you could disable 3rd party plugins temporarily to see if the problem remains. If not, some plugin is causing the trouble. If it's still there my guess is wrong.

It's a one-time setup thing, probably just as easy to disable it via admin isn't it?

@Matt Beckett The problem with disabling it via admin is that I sometimes forget to disable it after a fresh install, then when I encounter some problem and then spend some time debugging and finally found that I need to disable this theme. Therefore, could there be a better approach for the developer or user to choose a theme, for example in a config file like some other framework?

In your custom theme you can set aalborg as a conflicting plugin with the manifest.xml.  That will mean you *can't* enable your theme until aalborg is disabled.  Then you'll never forget.