I had exactly the same problem. I guess it is because Apple changed something on iCloud Drive. So you have to use another Syncing method like Dropbox.
Therefor you have to uninstall the app using AppCleaner https://freemacsoft.net/appcleaner/ but copy the "sync_default.walletx" file from your iCloud Drive first to your Desktop. Then just drag and drop Enpass into Appcleaner and uninstall it. Then reinstall Enpass and open it and choose "I'm an existing user" or something like that and than choose "from Backup file". Then you can change the syncing method to e.g. dropbox in the settings window.