Update: After testing with the help of Space Guy and Ephialtes...
It turns out that deleting prefs.cs and config.cs does nothing after the first time i join a server. If i rejoin, that GUI comes up again. Also with a fresh copy of Blockland, this problem still occurs.
And if it helps, I'm running Blockland from my Applications folder. I run Blockland on Mac OS X 10.5.8