in_array() expects parameter 2 …class-wp-gallery-metabox-admin.php on line 67

I did a quick workaround which is working for now:

if ($post_type == ‘gallery’) {
add_meta_box(
‘gallery-metabox’, ‘Gallery’, array($this, ‘gallery_meta_callback’), $post_type, ‘normal’, ‘high’
);
}

This will work probably forever on this one site, but I will put my actual fix when I have time to fully debug and re-write.