The upper volume button ist stuck. If I try to start the phone it always enters into the reboot menu, I guess because the button is stuck (“;pressed”) the hole time. I tried to disassemble the upper part of the back, but I cannot reach the buttons.
With reboot menu you mean the recovery menu, so from there you can reboot to system or do a factory reset? If so, what happens when you select reboot to system? Maybe its a software issue and a factory reset helps (which means loosing all data not backed-up).