Problems upgrading to 0.6: upgrade failed (getLoginPluginName), now "Class 'Zend_Registry' not found"

At first, the automatic upgrade failed with:

Fatal error: Call to undefined method Piwik::getLoginPluginName() in /html/piwik/core/View.php on line 129

(this appears to be

Then I got:

Notice: Undefined index: timezone in /html/piwik/core/Site.php on line 66

Backtrace →
#0 Piwik_ErrorHandler(8, Undefined index: timezone, /html/piwik/core/Site.php, 66, Array ()) called at [/html/piwik/core/Site.php:66]
#1 Piwik_Site->getTimezone() called at [/html/piwik/core/Controller.php:54]
#2 Piwik_Controller->__construct() called at [/html/piwik/core/FrontController.php:118]
#3 Piwik_FrontController->dispatch() called at [/html/piwik/index.php:59]

Fatal error: Class ‘Zend_Registry’ not found in /html/piwik/core/Updates.php on line 35

And now it’s just (at /piwik/index.php?module=CoreUpdater):

Fatal error: Class ‘Zend_Registry’ not found in /html/piwik/core/Updates.php on line 35

(it worked fine on another setup / server, using svn sources directly)

This could get fixed by restarting Apache / disabling eaccelerator.
I’ve also heard that this might be caused by an old Zend installation in the include_path.

I cannot say for sure, because I have no access myself to the server, just started the automatic update myself.