I’ve the same problem that has been noticed only this morning.
My quick and dirt workaround was to edit the file:
wp-content/plugins/issuu-pdf-sync/inc/class.admin.php
and comment the following lines (about 375):
if ( !isset( $form_fields ) || empty( $form_fields ) || !isset( $attachment ) || empty( $attachment ) )
return $form_fields;
As far as I could understand, WP 3.5 modified the attachment_fields_to_edit hook behavior and the $form_fields variable does not get passed anymore.
Commenting out those two lines made it work for me, but I don’t know if it’s going to break something else.
