Unexpected error

An exception occured while bootstrapping the application!

Zend_Session::start() - phar:///opt/otsn/waf/waf.phar/library/Zend/Session.php(Line:479): Error #8 session_start(): ps_files_cleanup_dir: opendir(/var/lib/php5) failed: Permission denied (13) Array

Stack Trace:
#0 phar:///opt/otsn/waf/waf.phar/library/Nstein/MVC/Bootstrap2.php(73): Zend_Session::start()
#1 phar:///opt/otsn/waf/waf.phar/library/Nstein/MVC/Bootstrap2.php(29): Nstein_MVC_Bootstrap2::initSession()
#2 phar:///opt/otsn/frontend/public/index.phar/public/index.php(53): Nstein_MVC_Bootstrap2::boot()
#3 /opt/otsn/frontend/public/index.phar(1): Phar::webPhar('3s-frontend.pha...', 'public/index.ph...', '', Array, '_phar_rewrite')
#4 {main}