My Bad!
As it turns out, the URL indeed needs to be in Samba format, to be accepted at all!
You might check the function that validates this in
http://yourNAS.local./web/function/localbackup.js
Which means, you need to enable SMB File sharing on your Mac.
However, I still fail it to sign in and validate the source folder:
Maybe the path to my shared folder shall look different. I really haven't found any compelling reasons to play with SMB so far