This is a very, very common problem with Samsung Cellphones.
Have a look here:
The general thinking on this problem appears to be that you can't do anything while the applications in the system tray are running. If you right-click and close those, some people have had success. If that doesn't work for you... well, the rest of the solutions are very technical in nature, and you should do some reading on the search provided before attempting any of it.
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