I had a similar problem syncing to iCloud. Having followed your advice above, the error messages no longer appear and the synchronisation "appears" to have succeeded, i.e. when I press the "Sync Now" button, the message "Last Synchronised: a few seconds ago" appears :-)
However, when I open my iCloud drive using the Finder on my Macbook Pro, I do not see any data - it appears that nothing is stored there by Enpass. There are also no hidden files :-(
I changed one of my passwords on my Macbook and the change was visible wiht the Enpass app on my mobile phone, so the synchronisation via iCloud works :-)
Why can I not see the file (Vault) on my iCloud Drive?
Device: Macbook Pro from 2018
MacOS: Mojave, Version 10.14.6
Enpass 6.5.2 (726)