THIS WORKED FOR ME, and EASY:
Go to the Samsung store, this is an app in your list of apps. I'd never even used it before...
Once there, look for the app SecureFolder (do a search).
When it comes up just click on the REFRESH button or maybe it's a download button or maybe it means to re-download. It's a button that has a spiral circle with an arrow like in Windows would mean "refresh".
I clicked on that and it re-installed the app and solved everything, my pictures and documents and password remaining in place!
But the question is, "WHY is the Samsung firmware breaking our securefolder app? And are they going to fix this bug? How do we report it to them so they can fix it?
Having the same issue, restarted and nothing changed, when I search the Google Play store it says that it's already installed. Yet it is still not showing in the list of apps.