after playing around with this issue i finally managed to get the fullscreen to fully belong to the application again. yet, this solution did not work since i had huge problems with the OpenGL ES application after switching back. sometimes it would work and sometimes it wouldn't. i believe the problem lies in bugs in the OpenGL ES driver on the X50v but that is just an educated guess.
so i did what Thierry suggested and closed the window (which btw can be done without closing the context under EGL, so this causes no problems) and recreate it lateron (rebinding it as a new EGL surface). this works perfectly and seems to be the cleanest solution.
thanks for all suggestions,
Daniel