One last thing to add, which I discovered after applying the latest DSM security update tonight: you must be logged in to DSM in order to be able to see the full server details in the Duplicati web interface.
I had it open in a separate tab, and after rebooting the NAS I could only see a blank Duplicati server with no scheduled tasks, until I switched to my DSM tab and logged back in. Makes sense when you think about it since the web interface is going through Synology’s front-end proxy, but a trap for young players if you’re not expecting it.