Fatal error: Uncaught exception 'Zend_Session_Exception'

I’d like to install piwik on my server. Its a ubuntu 8.04 with all updates done. Recent apache, recent php. recent libzend I follow the installation-instructions which is: download - unzip - point your browser there and then I get the following error:

Fatal error: Uncaught exception ‘Zend_Session_Exception’ with message ‘session has already been started by session.auto-start or session_start()’ in /data/apache2/virtual/piwik.goldfisch.at/piwik/libs/Zend/Session.php:462 Stack trace: #0 /data/apache2/virtual/piwik.goldfisch.at/piwik/core/Session.php(70): Zend_Session::start() #1 /data/apache2/virtual/piwik.goldfisch.at/piwik/index.php(44): Piwik_Session::start() #2 {main} thrown in /data/apache2/virtual/piwik.goldfisch.at/piwik/libs/Zend/Session.php on line 462

Actually I dont have any clue how to fight this problem. While I consider myself an expert in servers I’m no expert in php and less than a beginner in Zend.

Any tips welcome …