'Menu' and 'Home' buttons are gone.

'Menu' and 'Home' buttons are gone. This is not the first time I encountered this, but for now I am unable to reproduce this behaviour.

If I recall well, if just apprearsed when opening the mail app, but not mater what I did or what other app I openend, only the 'Back' bottom key was displayed. After the screen turned off, the behaviour was 'normal' again with all three bottom keys.

Please find 3 screenshots and a logfile. Hope that helps.