Just chiming in here to confirm that my wife lost access to her vault on her iPhone at version 6.8.3. She also updated her desktop app and now she has no access there either. She uses OneDrive for syncing the data.
I use exactly the same versions myself but no problems for me -- weird.
Enpass app is rarely updated and I am very disappointed -- your testing should have caught something as obvious! Please update the app ASAP.
Solved by this support article: https://www.enpass.io/support/how-to-setup-sync-with-your-box-cloud/
In short: I couldn't do it with Box directly, used their WebDAV integration to do it.
Problem described in title. Using macOS Mojave, latest update. Enpass 6, latest update.
Created a secondary vault and wanted to sync it to my Box account (free tier). Tried 3 times, same problem every time. I can see Enpass session in my Box profile -- successfully granted access to it.
Any ideas?