Well, I'm also having a very similar issue but I'm using Dropbox. Using Enpass on:
Windows: Version 6.5.2 (724)
MacOS: Version 6.5.2 (726)
iOS: Version 6.6.1 (528)
Android: Version 22.214.171.1249
All of those tell me they're the latest version when checking for updates. Sync seems to be working for Windows and Android but not for iOS and macOS.
I disabled the sync when they started failing with the intention of enabling again but now the devices can't connect to Dropbox anymore. iOS says "Error while enabling sync". macOS says "Could not verify your credintials.Please log in again to sync your data" (misspeling is as it shows in the message and of course I already tried to login several times to the same error).
Is the fix you mentioned above for Vault sync in general or is it specific for Onedrive (in which case I might as well open another topic)?