I upgraded the WordPress SEO to the latest version 126.96.36.199
I drive WordPress 3.4.2
By Activating the Plugin, the editor in page/post editor is blank. I can´t switch between visual and html. No other plugin is causing the problem. I deactivated all plugins, activated again, plugin by plugin. The WordPress SEO is the reason, why all quicktags disappears.
Switching to a standard theme didn´t solve the problem.