Thank you all! George, there is no polarization filter on the lens, removed it for these pics... But I noticed what you have noticed indeed...
Checking the aperture blades and I can't see any irregularities in closing
. But I noticed some (ahum) fat and dust on the back side of the lens. Removed it, but I can't test how the lens is working right now, because there is no sun (and I've given away my camera to a friend, who's is considering buying a Pentax). But I guess that is what caused the irregular star shapes....