Before I explain my problem let me say I read the Kubuntu thread with the same error and it is not applicable because I am not out of memory.
I run Ubuntu 8.04 and had been enjoying ussing 0.3.0 with gimp 2.4 fine for a while. I'm no linux wizard, but I am able to at least install 2.0 of the lib and 5.0 of the plugin. I am resizing a small image experimenting with the new rigidity map and it consistently gives me "Plug-In 'Liquid rescale' left image undo in inconsistent state, closing open undo groups."
It seems to only happen when I use a seam mask AND a discard mask.
I'm kinda done experimenting with the rigidity mask until the problem can be fixed, but I will update if I have problems with other features.