My last post was really too long, I guess.
After some days now I'm totally sure that the problems I'm facing are due to something wrong with the proximity sensor.
If I disable it with the command kindly provided by wolke some posts ago my phone doesn't lock up anymore even without rebooting the phone for days.

Do you guys have any suggestions about the right positioning of the proximity sensor? Is there really a correct alignment to take care of?
I just would like to see if there's something I still can do before ordering a new sensor...