Downgrading an X server is not trivial to do - This comsol is obviously
older software, or at least it's built against older X libraries. If
you're running the latest and greatest ubuntu and like it, then you
might want to try running an older ubuntu, or debian, in virtualbox. If
you've no great investment in this system yet, then you might just
downgrade to ubuntu 8.04 or whatever version it needs to be for that
software to work