I managed to fix my problem (syncing with Dropbox):
- I couldn't connect to the App Store previously, so that option was out. I decided to address that issue by upgrading to Mac OS Mojave.
- Once the OS was upgraded, I removed the standalone app (it still didn't connect) and deleted everything except for the backups via the Enpass app
- I then downloaded the app via the App Store
- I authenticated via Dropbox, got the redirect prompt and this time Enpass synced with my Dropbox account!
@iota – Depending on your situation, you may want to try something similar. Hope you get your problem sorted!