An error occurred Call to undefined method Piwik\Menu\MenuAdmin::addSettingsItem()

Since upgrading to 3.0.0-beta3 I get the error “An error occurred” “Call to undefined method Piwik\Menu\MenuAdmin::addSettingsItem()” and am not able to get further any more.

Who knows what to do?

Nearly the same here after an update from beta2 to beta3:
An error occurred
Call to undefined method Piwik\Menu\MenuAdmin::add()

It seems that after deleting the config.ini file and then following the reinstall everything works until I reactivate the custom optout plugin. After that the error returns.

is there a solution to that problem?

This thread is quite old. Iirc the plugin version was not fully compatible with the new version of Piwik/Matomo. Meanwhile it should be compatible again. So updating the plugin should solve that issue