I'm building a website with my friend who bought WP User Frontend PRO version couple of days ago. I'm a developer of this website. We had a same problem with Free version previously and we can't manage to work it out. There are two issues and I think they are connected in some way:
1. After a success payment (not in test mode) the payment result page is blank.
2. Transaction record on database always shows user_id as 0 so user after buying a package can't add new posts using a plugin.