Could you please supply more information to help characterize the problem? For example, can you see how many files did make it onto the NAS (look for names containing dlist, dblock, or dindex), or did none of them make it? It sounds like something made it (was it the exact number reported missing?) because you deleted.
Creating a new backup job shows some steps you probably used. What “Storage Type” did you set, and did you click “Test connection”? That’s a good initial test, but there might be others yet to come to test systems.
When does that result occur relative to the start of the backups? At the very start of the initial backup (which seems unlikely because it shouldn’t be missing files while newly empty), at the end of the initial backup, etc.?
The default Duplicati install is as a Tray Icon for that user. Can you confirm that you used default GUI install? Installing as a Windows service is possible too, but takes manual command work and adds some complexity.
Although 2.0.3.3 beta should be doing better than you’re experiencing, I’d note 2.0.4.5 beta is available too.
Is the Duplicati Home screen entry for this job showing Last successful backup: Never? If so that may actually simplify early debug because it won’t be necessary to reset, i.e. does it just keep failing every time?
Please add backup options –log-file=<pathname> and –log-level=Information to see names of missing files, and also the names of files that were sent (look for Put operations). If Put, they should still be around later. Obviously you’re having some sort of disconnect between the Put and the view later (by a List operation).