What is causing this flare?

[Alan]

The lens is well over 2 years old.


Typically, on these night shots, I leave the filter off. But, it was on for this event. Could that have contributed to it? Usually, what I see when the filter is on, is less sharp point lights, but not this sort of "light sabre" flare.

[Gian Luca]

I always take the filter off if I have a light direct into the lens, as I always noticed more flaire with the filter. The quality of the filter is very important as well