Exactly same error as in issue #109

When I put the middle wither skull, it doesn't get placed. When I place it a second time, the entire structure (4 soul sand + 3 wither skull) disappears and a single wither skull appears in the ground, right where the bottom soul sand was.
When I try the command, I get the same error: "An unknown error occurred while attempting to perform this command" but without crashing. The error appears in the chat window and nothing happens then.

I even tried to do a fresh install (since I tinkered a bit with some extra mods in my other Bounds instance) and I get exactly the same issue. Any clues? Some misconfiguration, mod incompatibility, etc?

I'm using MultiMC 0.6.2 in Ubuntu. Can't try in my Windows partition for now since Minecraft was performing ultrabadly and I uninstalled it.