I found the mistake.
It is not on the Youtube Lib, but on the adjusted Orientation.
When the video is started:
p.SetScreenOrientation (0)
And when closing the video I have set:
p.SetScreenOrientation (1)
Thus, after changing the Orientation, the Layout is reloaded but this may not happen ...
If I omit the Orientation, the back function of the Youtube Lib works quite normally.
EDIT: /close.
I have solved it as follows:
SetActivityAttribute(Main, android:screenOrientation, "portrait")
SetActivityAttribute(Main, android:configChanges, "keyboardHidden|orientation|screenSize")