oscb

Problem solved. I will explain why it happened and how to solve it if you find yourself in the same trouble:

I was using this framework for taxonomy metadada. Even though it is an awesome code, it has a function that causes trouble: check_field_wysiwyg()

The thing is that it is enqueueing jQuery everywhere it finds a WYSIWYG textarea and that includes the “Add Post” area. This causes trouble passing the ID of the images to the Featured Image Area on WP, so it saves just “-1”.
Just comment the wp_enqueue_script(‘jquery’); line on the code and everything should be going on just OK!