No write permission for knx.cfg

Hey there,
i just installed installed openHab 2.2.0-1 on a Qnap TS-251. The installation worked and I can access Openhab.
I created a user account in the NAS with RW permission to the share where openHAB is installed and I installed the KNX binding.
Its possible to access the conf folder and edit and save for example the items file. But its not possible to edit and save the knx.cfg file. Does anybody have an idea whats wrong?

Problem solved! I had to make the changes with the admin user.