After some more research I got the solution for myself. Im a noob - the flashplayer was not updated.
Here are two maybe useful hints i needed for mediaelement.js:
1.
The flashfallback works only if there is also a <source>
Element for some other format in the <video>
element and not only the <object>
element for flash.
2.
The fullscreen mode in IE 8 works only, if the <object>
gets following parameter: <param name="allowFullScreen" value="true" />