Since a while i have the problem that the standbyscreen didnt work. All the night i have the actual screen on. (Configuration standby screen after 15 s)
Also if i put the screen down, the screen doesnt get off for sign of approximity-sensor.

Approximity sensor works on Phone Calls.

I think the configuration file for X has a problem. Someone can guide me which file it is? I have another N9 (16GB) to compare the file.

Thanks in advance

You could try to change some of the display settings in order to overwrite the configuration file(s), e.g.:

Settings -> Device -> Display -> Stand-by-screen -> On | Off

This has helped me in the past when something related to the display misbehaved.

i did it.
Also change the time and back.

Not solved

I got similar issue after tuning status bar through n9 qtweak. The solution I found is easy and unrisky : Restore the Settings (in Reinit section of System setting. Not painful. I just lost the folders in homescreen.