When running from a service, you do not have the “user context”.
The way network drives work in Windows is that they are opened/mounted for the current user, and the login is authenticated using the current user.
When you are running as a service, you are connecting to the share as the local system (machine) administrator, which does not have access to remote shares.
It should work if you enter correct credentials in the destination setup, as Duplicati will then attempt to pre-authenticate with those credentials before connecting. As the destination is then opened/mounted it will not attempt to log in with the local system account.