Symantec System Recovery 2013 R2 Authentication Issue with NAS

Hi,

I imagine this question has already been addressed before but Google / Bing searches have lead me on a wild goose chase, so I'm posting this question here in hopes of an easy resolution.

We're using Symantec System Recovery 2013 R2 and we just installed a new Qnap NAS.  The environment is mixed Win7 / Win8 and some of the SSR clients have no issue authenticating with the NAS so that they can store backups there.  Roughly 35% of the client machines, though, can't authenticate correctly, no matter what login I use (e.g. domain\username : password, localusername : password, etc.).  Since it works on most of the machines, I feel confident my domain accounts and local accounts are setup correctly.  I even synced the passwords temporarily so I could side step the possiblity of a mistyped password.  The clients that won't authenticate with the new NAS will authenticate with an old failing Lacie NAS with a default local account of admin : admin, but they simply fail with every account I've tried with the new NAS.  I can't detect anything different between the windows 7 boxes that fail and the ones that suceed (same with the Win8 boxes).

Any clues would be appreciated, since I need to clean up our backup architeture as soon as I can.

Thanks.