Media Uploader issue

I have WP MU and I have a page on main domain from where blog admins can update/manage their pages/posts. so it looks like: http://www.MainDomain.com/members On that page I am check if user is blog admin and I am gathering infos I need and doing this:

switch_to_blog($userblog_id);
wp_enqueue_script('post');
wp_enqueue_media( array( 'post' => $post_id ) );
wp_editor( $post_content, 'post_content', $settings = array(textarea_rows => 15, media_buttons => true) );

$userblog_id is actually ID of the blog that user is assigned to. $post_id is ID of the post that user is editing on member page.

Now when blog admin clicks on Upload Media button it shows media upload popup, but the media gallery is always empty and when admin tried to upload something it is always failing with this error “An error occurred in the upload. Please try again later.”

I saw that MainDomain.com/wp-admin/admin-ajax.php is called when I click Add Media button and that json response is: {“success”:false}

Now when I am logged in as superadmin – it works properly.

Does anybody have an idea what’s wrong here ?