Error after Upgrading to 2.9.1: Fatal error: Undefined class constant 'Piwik\Tracker\GoalManager::TYPE_BUYER_NONE' in /www/htdocs/xxx/core/Singleton.php on line 37

Hello

i just tried to upgrade to 2.9.1 automatically via update manager - but now i just get this error:

Fatal error: Undefined class constant ‘Piwik\Tracker\GoalManager::TYPE_BUYER_NONE’ in /www/htdocs/xxx/core/Singleton.php on line 37
A fatal error occurred

The following error just broke Piwik (v2.9.1):

Undefined class constant ‘Piwik\Tracker\GoalManager::TYPE_BUYER_NONE’

in

/www/htdocs/xxx/core/Singleton.php line 37

Troubleshooting
Follow these steps to solve the issue or report it to the team:

If you have just updated Piwik to the latest version, please try to restart your web server. This will clear the PHP opcache which may solve the problem.
If this is the first time you see this error, please try refresh the page.
If this error continues to happen, we appreciate if you send the error report to the Piwik team.

Further troubleshooting

If this error continues to happen, you may be able to fix this issue by disabling one or more of the Third-Party plugins. You can enable them again in the Plugins or Themes page under settings at any time.

If this error still occurs after disabling all plugins, you might want to consider uninstalling some plugins. Keep in mind: The plugin will be completely removed from your platform.
ExampleFeedburner uninstall
SecurityInfo uninstall
VisitorGenerator uninstall
Referers uninstall
PDFReports uninstall