I got an answer back from Zeiss, which seems to be an honest one.
Apparently onion rings are normally indicators for aspherical elements, showing the structures of the manufacturing process. They also say that visibility and intensity of these structures depend on various factors in the scenery, such as the intensity of the light source compared to the surroundings, the exposure, the spectral built-up, the amount of "unsharpness" and many other factors. They also mention that some bright light sources in the out-of-focus areas may be clean, due to light intensity (saturating the sensor).
So, without starting a new series of unfocused shots of all the various light sources I have available at home I think it is fair to conclude that this has to do with the lens. But! I made lots of shots where I deliberately used very fast shutter speeds at low ISO, to make sure I didn´t saturate the sensor. And in the cases where I had a clean light source, being a candle, a halogen point source or a gas filled bulb, I did NOT get onion rings.
So, I am still puzzled.
Yes, it's very strange. To quote one of my favorite songs, "the problem with mysteries is they're so mysterious."