I gave it a last try and looked for proximity sensor libs and drivers.
(as root) "ls -r /* | grep prox" showed zero results.
I hope this is the reason for the problems. I flashed the the 40.* version three times, so i have no clue what else i could do to install the proximity sensor software on the N9. Maybe I could init a regular update. But i don't know how, and i already have the latest version running on my N9.