I have a GFCI breaker on my main panel, with several outdoor deck lights on that circuit. They work fine until I turn on a track light in an indoor room, which is on a separate non-GFCI circuit, at which time the original GFCI circuit trips and will not reset until I turn off the track light.
What would cause this?
It sounds like you have the neutrals (of both circuits) connected together somewhere outside of the panel.
Trace out the wiring of these two circuits, if possible.
A neutral backfeed is possible. But I would look at the track lights themselves and see if the light fixtures are ok. Remove them one by one, from the track until you find the one that is causing the problem.
USA The situation you describe is not possible if the circuits are completely separate. I agree with the others, you might have a connected neutral. Have a licensed electrician check it out.