I see this on my screen - and nothing else
in ‘/path/to/piwik/core/FrontController.php’ at the line 263
#0 Piwik_ErrorHandler(…) called at [/path/to/piwik/core/FrontController.php:263] #1 Piwik_FrontController->init(…) called at [/path/to/piwik/index.php:52]
please try to reupload all files?
I had the same problem. The solution was to reupload config/global.ini.php from: http://dev.piwik.org/svn/trunk/config/global.ini.php
Maybe your database needs an update afterwards and you should then be directed to the login screen.
That seems to be a similar problem as for the users that end up with a blank screen after the update.
Solution from: 301 Moved Permanently