Pandur23

I ran Chunkster v0.3 (http://forums.bukkit.org/threads/admin-chunkster.8186/) on the world and now I can join again

I have the same situation. As far as I understand, a redpower machine stopped working. In your case, this seems to be a Filter (see the first Details block). You could enter the old world (coordinates are provided), remove the filter, and try again to convert it.