Am I correct in assuming that you’ve just switched to Ubuntu 16.04, and that these commands worked fine before on the same data running on a previous version of Ubuntu? If so, then the reason is most likely related to the particular version of Eigen that Ubuntu ships on 16.04, as in this post.
Can I just check whether your MRtrix install is up to date? We committed some changes a couple of weeks ago that were supposed to fix this problem, but it’s likely we might have missed some edge cases, which might not have been picked up in testing. If your install is more recent, then we’ll need to investigate these particular errors on our side. Maybe you can update with git pull; ./build and try again just to make sure…?
If that didn’t help, then the simplest fix is to download the stable release of Eigen - you’ll find instructions to do this here.