If you get the following error when logging into squirrelmail:
ERROR Error opening ../data/default_pref Default preference file not found or not readable! Please contact your system administrator and report this error
ensure that you have not enabled safe_mode globally in your /usr/local/lib/php.ini file. (set it to off). If you want safe_mode on your domains, then enable them from:
Admin Level → PHP Safemode Configuaration