Unable to back up to NAS

Hi

We are running Backup and Restore 11 on a Windows 2003 server. We've been backing up to internal drives successfully for years. However, due to the 2TB limit on Windows 2003 32-bit, we've just added a 3TB NAS unit to the network. The unit is recognised in Windows, but Acronis refuses to back up to it. The unit is a QNAP TS-269L.

First of all I tried setting the address to \\192.168.0.22\Public and entering the admin user and password. That failed with an error message. So I mapped drive F: to \\192.168.0.22\Public and tried to use drive F:. Again, it failed. I then created a vault on \\192.168.0.22\Public, added the admin credentials and tried again. Failed. There most recent error is:

ProtectCommand: Failed to execute the command.
Additional info:
--------------------
Error code: 41
Module: 307
LineInfo: e6792a5ee190dd9e
Fields: $module : agent_protection_addon_vs
Message: ProtectCommand: Failed to execute the command.
--------------------
Error code: 1
Module: 149
LineInfo: 15b308bfdeeeb9e
Fields: $module : disk_bundle_vs
Message: Invalid parameters were specified.
--------------------

Firstly, what is the best way to back up to the NAS. It is not currently part of the domain. By IP? By mapped drive? By personal vault?

Depending on the method, what might I be doing wrong? I can read and write to the NAS fine with windows, but Acronis B&R just won't have it. :(