For the moment it is not possible to install Piwik on some Linux machines, if the session.save_handler 'mm' (Memory) is used. The problem lies in Session.php, where the session is initialized only for 'files'.
As for type 'user' a manual switch to 'files' is made, I suggest doing the same for 'mm'. This worked in our case.
(In ) fixes #1515; refs #806; mm is not supported