Same/similar issue with Enpass 6.8.0 on Mac (OS 12.3.1 Monterey), iPhone (iOS 15.5) or Windows 10. Old previously authorized and sync'd vaults (created by earlier versions of Enpass) synchronize OK, but new vaults cannot be sync'd to the server via v.6.8.0 running on any of the platforms. I suspect the issue is relaying the username or the password (probably the latter) to the server.
This happens with WebDAV running on Synology DS418 on DSM 7.1 and WebDAV Server 2.4.3; downgrading the server did not help. Synology troubleshooting could not find anything wrong with the server and in any case the WebDAV server could be successfully logged in via CyberDuck.
Downgrading Enpass to v.6.7.4 solved the issue on the Mac. Unfortunately, the iOS app can no longer be downgraded and the issue persist on the iOS (iPhone). I'm wondering if Enpass can make the previous version of the iOS app available as a temporary workaround.