Zend_Session / Zend_Auth randomly throws Error Message ps_files_cleanup_dir: opendir(/var/lib/php5) failed: Permission denied (13)

I'm currently working on a new Application using (among other things) Zend_Auth but, for whatever reason, this Error Message is showing up at any location totally randomly (or so it seams)

Zend_Session::start() - /home/hannes/workspace/develop/library/Zend/Session.php(Line:480): Error #8 session_start() [function.session-start]: ps_files_cleanup_dir: opendir(/var/lib/php5) failed: Permission denied (13) Array

• #0 /home/hannes/workspace/develop/library/Zend/Session/Namespace.php(143): Zend_Session::start(true)
• #1 /home/hannes/workspace/develop/library/Zend/Auth/Storage/Session.php(87): Zend_Session_Namespace->__construct('Zend_Auth')
• #2 /home/hannes/workspace/develop/library/Zend/Auth.php(91): Zend_Auth_Storage_Session->__construct()
• #3 /home/hannes/workspace/develop/library/Zend/Auth.php(141): Zend_Auth->getStorage()
• #4 /home/hannes/workspace/develop/xxxxxxx/application/controllers/AdminController.php(10): Zend_Auth->hasIdentity()
• #5 /home/hannes/workspace/develop/library/Zend/Controller/Action.php(133): AdminController->init()
• #6 /home/hannes/workspace/develop/library/Zend/Controller/Dispatcher/Standard.php(262): Zend_Controller_Action->__construct(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http), Array)
• #7 /home/hannes/workspace/develop/library/Zend/Controller/Front.php(954): Zend_Controller_Dispatcher_Standard->dispatch(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http))
• #8 /home/hannes/workspace/develop/library/Zend/Application/Bootstrap/Bootstrap.php(97): Zend_Controller_Front->dispatch()
• #9 /home/hannes/workspace/develop/library/Zend/Application.php(366): Zend_Application_Bootstrap_Bootstrap->run()
• #10 /home/hannes/workspace/develop/xxxxxxx/public/index.php(26): Zend_Application->run()
• #11 {main}