check wires

I'm playing with The low power door/window sensor and noticed that from time to time the server is not responding when I switch a button to simulate the opening/closing of a door. I'm not sure if the server is really freezing. It's also possibel that there is no communication between the 2 radio's. Is there a way to check the communication by using leds for example to see if both modules are really sending/receiving?

Yes, actually there is a way to check communications:
yveaux.blogspot.nl/2014/07/nrf24l01-sniffer-part-1.html

regards

Finally success! I changed the channel from 90 to 76 and it's now stable as a rock. I'm happy! But it's still not clear to me how to pick the right channel.

I discovered the same almost. Mine work on 95, but I reaaly don't know why.
In the config file is there two differnt values, one at 95 and one at 90.
But 90 doesn't work, but p5 does.

Which config file do you mean? I've changed the channel in the webclient only.