After launch of Enpass and unlocking the vault, the Synchronization process immediately starts. In my case it syncs to iCloud. But it never finishes, it never stops. Although I restarted Enpass several times and gave it multiple hours, it never stops syncing. 

It is also not possible to cancel the Sync in order to select a different sync target.

Any ideas?

I got the same problem. iOS Devices are working fine.

Somewhere here I read about this solution how to stop syncing and set it up correctly again:

Disable all network connections
Restart Computer
Enable network ... I cannot remeber the exact procedure thereafter, when to set up syncing again. There was another restart involved.

If you browse the entries here, you will find it. It appeared a few days ago.