just for information , I got exactly the same problem : network OK when setting up an uPNP share but I Got "network is not connected" when reading this share.
The solution is also OK for me : relaunch XBMC without rebooting Apple TV !
It means XBMC must be launched twice for network access OK ?
Yes, your solution works.
I also use wired connection and I have to launch XBMC twice after each Apple TV reboot or NAS reboot for XBMC detects my smb shared folders and my UPNP server.
Note: update ssh to fix wifi problem and trouble persist.