The old VCC/2 line, which I originally made with a second opamp has been replaced with a 100K resistor pair. Not the best solution because now it hums even louder. The grounds and the opamp bias have been properly decoupled this time (still hum). Some of the capacitor values have been lower in cutoff frequency (still hum). I tried placing a resistor over the input to remove the hum. That works if the resistor is ~680 Ohm. Yet in that case, if there is an input signal, it is barely amplified (no hum, no signal). Putting the entire thing in a metal box did not do a thing. So no pickup of magenetic fields (still hum). Replacing the powersupply with a batterypack resulted in >huray< no hum ! So it is most certainly a power supply problem. Hum removal is a non trivial problem. Grmpf.