That looks to be a nice scanner, the one our client has is a Fuji Xerox DocuScan C4250.
I got my service manager to get the manual for your scanner and it appears that it should work as you want but...
Since your scanner is very windows orientated and the DNS-345 uses a linux based OS there may be an conflict between the CIFS implementations of the units that is causing the problem.
One thing you could try is using FTP to communicate between scanner and nas - at least that is an international standard. It does require you to setup the ftp server on the nas and creating a profile to use it on the scanner.
To make life easier it is a good idea the give the DNS-345 a fixed IP address.
If it works with FTP and does what you want and there are no other problems created on the network by having the FTP server available then, if you were our client, I would say the problem is solved. If, on the other hand, running the ftp server is a problem then we need to find out what the problem is between the CIFS implementations and try and fix that - not an easy job not having the scanner to work with.