Why does my 722.9 gearbox engage only second gear and reverse on 2005 Mercedes-Benz E500

For the last month or so would not change down until I stopped, switched off ign,locked the car, unlocked, and then start again, ok for a few days. Now I can only get 2nd and reverse. Car has only done 54000 miles. 7 speed box. Diagnostic fault code P0717

Asked by for the 2005 Mercedes-Benz E500
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