Same again. I have 11.04 xubuntu and tried the 10.04 version of iplist, but I get the exact same error with or without trying the gui. However, I had previously tried another pc with 11.04 ubuntu (full version with gnome) and iplist and its gui seemed to work okay, at least without errors, though I don't remember if I tested the blocking. maybe there is just some component from gnome that I need?
Even if this project isn't maintained, the lists themselves get updated. Or should I just try a different solution?