Hi, I had the same problem with my J7 Neo. But I think I figured out how to fix the problem. First I apply the hard reset. Then I go to the Galaxy Store, which is the icon pre-installed on the phone and look for Secure Folder app, there you will find an update. Finally, applied the update. At the beginning It seems that it's not working out because the OPEN button doesn't work. Close all the windows and open the secure folder from the toolbar. It worked with my phone. I hope this helps you fix the problem.
The April 1st 2019 security update broke the Secure Folder feature on my Samsung J7.
Factory reset didn't fix it. The latest system firmware is bad.
... View more