After upgrade to Piwik 1.6 many users are reporting "Unable to start session" messages. It seems to happen when the piwik/tmp/sessions directory is not writable.
INstead we should detect this before trying to start the session and if the directory is not writable, return the standard piwik error message showing the "chmod" help command.
See also a report in: http://forum.piwik.org/read.php?2,82920
Maybe the whole tmp/ folder should be checked more often?
Definitely a problem across the board...
problem writing temporary file '/var/www/vhosts/..../piwik/tmp/templates_c/wrt4ebc5ba68a6af' ``` http://forum.piwik.org/read.php?5,82929
(In ) Fixes #2717
Solution to problem "session has already been started by session.auto-start or session_start()"
Set session.auto_start = Off in php.ini