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 #2 session_start(): open(/var/lib/php/sessions/sess_mihmrkttjb08aroqi9fi86i8e7, O_RDWR) failed: No space left on device (28) Array phar:///opt/otsn/waf/waf.phar/library/Zend/Session.php(Line:489): Error #2 session_write_close(): open(/var/lib/php/sessions/sess_mihmrkttjb08aroqi9fi86i8e7, O_RDWR) failed: No space left on device (28) Array phar:///opt/otsn/waf/waf.phar/library/Zend/Session.php(Line:489): Error #2 session_write_close(): Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/lib/php/sessions) 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}