It just doesn't work this way! Forgotten, but very important hints:
Before step 4: Do step 5 to 7 and remove the back case. Then clip off the one clip marked by a red circle in step 4. Disconnect the little red/black cable on the lens side, right next to the "2x optical zoom" writing on the case. Now the front case is still connected by a ribbon cable: In step 6 you can see the ribbon cable, which holds the back cover (and which you have just disconnected). Watch the small white connector marked in the picture with a red square. Right below the red square, you see another white connector (with no cable plugged in on the picture). This connector has no lock, like the one above, just pull the cable out. Now the back case is free!