What’s odd is that even though ExpandInputSources is on the sources side, this message appears to be NAS-related (glancing through Google of “The specified server cannot perform the requested operation.”).
Might there be any way e.g. via mklink that something in the “Source Data” config got sent to a NAS area? Possibly you could try different subsets of your current sources (easily done via Commandline or Export).
Is Duplicati running as a user at login, or as Windows service? Services have SMB authentication issues.
You could try setting up –log-file at –log-file-log-level=verbose or higher. Even though ExpandInputSources doesn’t announce every expanded result, it does issue some of them. Maybe it will give clues about failure.
If you think it might be related to your NAS somehow, maybe there’s an error log there, or you could add its brand name to the Google query at top to see if anybody has seen a similar problem there in some way…