This means the user who is running the Duplicati service does not have read access to the UNC path you entered. Since it is running as a service, it is running in its own user session. It sounds like you may have permissions to access the UNC path while logged in as yourself, you likely have the credentials to access the UNC path cached in Windows Credential Manager so maybe you are not getting a logon prompt, but this UNC path may need credentials to access. Duplicati does not support username/password for a UNC path as a Source, it only has those options for a UNC path as a destination. Here is another post I made a while back for someone having similar issues as yourself, this is most likely what you will need to do in order to backup a UNC source that requires credentials to access: