I ran into this problem with Lion and I am sure more and more people will now, as it was released.
My Thecus NAS was working fine with Mac OX X 10.6 but with Lion more changes came along,
and now it is necessary to change a few things to make it work.

For those who are interessted, this also came up in the Apple Developer Forum here. 

Open a Terminal and run this commands:

sudo chmod o+w /Library/Preferences
defaults write /Library/Preferences/com.apple.AppleShareClient afp_host_prefs_version -int 1 

 Now restart your computer. Open a Terminal again and run the following commands:

sudo defaults write /Library/Preferences/com.apple.AppleShareClient afp_disabled_uams -array “Cleartxt Passwrd” “MS2.0″ “2-Way Randnum exchange”
sudo chmod o-w /Library/Preferences	

Restart your computer again and afterwards it should work again.