Can anybody explain this effect?

I think it's caused by the diaphram in the lens. Try some more shots with different apertures and if you can try another lens with a different number of leaves in the diaphram - should yield slightly different results - ie more/less points to the "stars". I took some similar night shots on a TLR over 30 years ago and I got 5 point "stars" on the street lights.

Vic.

I think xbow is right that the number of points is related to the number aperture blades. 18 and 9 respectively, in this case.

they are indeed a diffraction effect caused by a non-circular aperture. The effect becomes stronger the more you stop down, because the aperture becomes both less circular and smaller.

If you like the effect, stop down when there are strong light sources present and it will apear

if you don't like it, use a Neutral Density filter to achieve long exposures in these situations rather than stopping down.

Its caused by the light bending through the aperture. I guess you were stopped down around f/16 or so. Bring it up and the effect is less noticeable wider open.

well done - f/11 according to the exif data.