Ruby 1.9 and Sass encoding issues. (from ASCII-8BIT to UTF-8)

To complete @nex3 answer you can

Open the config.rb file and add:

Encoding.default_external = "UTF-8"

It looks like somehow your Ruby environment is set up to load the Sass source code as ASCII-8BIT, a.k.a. BINARY. This is probably because you're either passing the -Kn flag to the Ruby process, or setting Encoding.default_external to Encoding::BINARY somewhere in your code before Sass is loaded. Undo this, and Sass should work.

I don't know exactly the core of the issue, but it seems there were some problems with Ruby 1.9.0 (the version available on apt for this Ubuntu box) and encoding.

To solve the issue, I used https://rvm.io which is a helper to download and compile Ruby from Git, giving me the latest version. After running this, sass compiles fine and everything is swell.