@ts678 In default setting, service runs under SYSTEM account.
SYSTEM account act on network on another devices as COMPUTERNAME$ accout (with dolar sign)
You can share folder to this account and Duplicati in service mode can acces it.
Another way is change service account to something else.
@ts678
That’s interesting. I just tried that on Synology, sadly it did not work. Maybe I would need another syntax for the shared folder path. BUT you give me the idea how to do it on Synology.
I can mount CIFS folder and then select mounted folder as backup source - and it’s works 
Thanks 