@StefanoF I made a few more tests with the most recent code I have posted (which is the initial goertzel code).
a) test for phase jitter. Instead of reading the ADC, i just supplied a artificial 50Hz sine with an amplitude of 50 and an offset of range/2 into the algorithm. Result: constant 49.99993, which means that there is no phase jitter.
b) I connected the interface network, but not the sensor. Result: Output values between 0.005 and 0.03. Since the output value of 1Veff is about 240, that is equivalent of ~60µVeff, or about 1 mA through the sensor.
c) connect a battery with 1.5V to the input. Result: values of about 0.5 -> Noisy battery!
d) connect the SCT013 with no current. Value of 0.04 +/- 0.03. That's about 160µVeff of 3 mA (or 0.5 W at 230V) through the sensor. So it looks as if the sensor picks up some noise, but still ok.
e) Using an external ADC (mcp3201). Result: values of 0.05. Slightly worse than using the internal ADC.
Lesson learned: I will stick with the initial solution. The challenge is to proper shield the interface network and the cable from picking up unwanted noise.