Just to be clear I did not "short" anything to ground or PE. I uploaded two pictures of the bush-fix. The green wire is connected to PE and is simpliy taped down between two layers of insulating tape, right above the antenna. It works best if the PE wire is close to the outer edge. WIFI is still working well (my Access Point is 10m away + 3 walls). Powering via USB is not an option as I need a to measure independent of a common ground. It might not be perfect, but at least there are no more peaks above the noise level.
As already mentioned in your FAQ, there are some issues with noise/peaks on the oscilloscope channels of the OpenScope. In the example (OpenScope_Peaks.png) you can see distortions of up to 200mVpp. The test setup: Ch1: connected to DC1 @ 157mV Ch2: shorted to GND OpenScope is powered by a 5V USB power bank I noticed, that the distortions can be completely eliminated by touching the PCB antenna of the MRF24. It seems to work equally well if you tape down one of the insulated ground wires onto the PCB antenna. or keep protective earth close to the outer edge of the PCB antenna (e.g. between two layers of insulating tape).