Hi, I have this same issue and it has been driving me crazy!
I have one laptop (HP EliteBook 2530p, Vista Business 32 SP1, built in Intel WiFi) which is normally in docking station, i.e LAN cable.
I also have a HTPC (Vista Home Premium 32 SP1, D-link DWL-G122 HW ver C) wich is connected though WiFi.
My wife has a laptop (Dell M700, XP Pro, built in Intel WiFi) which is connected through WiFi.
I got the DNS-323 last week and my HP and my wifes Dell has worked fine, but the HTPC can see the DNS-323 and it can connect to the root of shared folder, but when you click on a subfolder exlopere freezes for ~1min and after that either say "folder is no longer available", or "you might not have access".
At first it worked fine and I could copy all files from the HTPC to the DNS-323, but when it was all copied it started acting like mentioned above. The HTPC has been acting strange lately, so I decided to reinstall it.
After reinstallation it worked fine again for ~2days, but now it doesn't work again.
I expect there is something with Windows update that is causing this, it might have happened when SP1 was installed (my clean install is early Vista w/o SP1).
I have been looking everywhere on the web to find information about this, also doing some tweaking and regedit, but no result.
I thought the problem was with Vista Home Premium, since my HP (Vista Business) works fine.
However, I started thinking it might be caused by the WiFi, so I diconnected the LAN cable from the HP, and then it acts exactly like the HTPC.
When I have the HP in the docking station it works fine, but with WiFi it doesn't work.
My router/DSL modem is a Thomson SpeedTouch ST585.
It should probably be mentioned that in the DNS-323 I have 2x1GB Samsung HD's RAID1, and I have ~20000 files, ~300GB. It has the latest FW (1.06).
FTP, AV server, iTune server, etc is turned off. I have tried both with user account/no user accounts, password/no password, but no difference.
I will try to get hold of another WiFi router and see if that will make a difference.
I will give this up now for some days of traveling, but please let me know what more information that is needed from me and I will try to assist.