I am also having this issue.
Every time I attempt to sync my icloud-backed vault on Windows, it will fail:
1. Click "Reauthenticate"
2. A new browser tab will open (I've tried both chrome and edge) to idmsa.apple.com for authentication.
3. Credentials are passed in with username / password OR passkey successfully.
4. Browser attempts to pass the assertion over to Enpass (browser is allowed) but Enpass just continues to say "Synchronizing..." while the tab sits idle
Enpass 6.11.9 (1915)