I found a bug trying to delete a relation between a post and a custom post type called "test".
I tried to delete the relationship directly from the post edit page, using the box called "fields table".
I selected the "Not Selected" corresponding to "This post belongs to"
This is the result of the ajax request:
"Passed wrong parameters"
I think the bug is in the function wpcf_pr_admin_update_belongs line 963.
#if (!empty($post_id) && !empty($post_type) && !empty($post_owner_id))
In the case of "Not selected" the $post_owner_id is always empty!