If it’s only the file default.m3lib you’re having problems with, I guess @JonMikelV is on the right track.
In case it still turns out to be a permission problem (I don’t expect that), check if the location on your NAS is stored in the Windows Credential Manager.
The current version of Duplicati doesn’t make use of the Credential Manager, but there are plans to use the CM for storing passwords in future versions. So if credentials for accessing your NAS are stored in the CM, Duplicati isn’t aware of it.