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?