In my /wp-content/themes/mobile_pack_base/functions.php on line 39 and 40 the path to group_detection.php still points back to the "wp-content/plugins"-directory, so my group themes are not active!
To fix this I've changed
$wp_mobile_pack_dir = WP_CONTENT_DIR . '/plugins/baap-mobile-version'; include_once($wp_mobile_pack_dir . '/themes/mobile_pack_base/group_detection.php');
$wp_mobile_pack_dir = WP_CONTENT_DIR . '/themes/mobile_pack_base'; include_once($wp_mobile_pack_dir . '/group_detection.php');
Note: I'm using a customized mobile theme and "Template: mobile_pack_base"