I get a test error too, but the test file actually is written to the network storage despite the error.
My problem (other than that) is that the camera won't actually save the network storage settings. I have three 2332L cameras running two different firmware versions among them, and not one of them will save the settings. I can't find any combination of real or fake settings that any of these cameras will save. I haven't had any trouble saving FTP, mail, or SD card settings (and I only have two of the five maximum servers configured, so I'm not hitting the limit). I have no idea why this doesn't work.
Could I ask you to share your NAS settings that were successfully saved?
And since the camera is successfully writing to your NAS elsewhere, does it really matter if you're getting the test error? Is it putting the test file out there (it's just a text file)?