In case the browser timing out isn't the problem, here's one other idea: You might not have PEAR installed, or PHP might not be able to include PEAR.php.
Also, if you've got logging enabled (w/ "logger_message[] = file" in your config.ini.php) you might be able to find more information in the Piwik log files.
Also, if you've got logging enabled (w/ "logger_message[] = file" in your config.ini.php) you might be able to find more information in the Piwik log files.