I assume "light 1" is wired as a two-level fixture where the red and black wires each control part of the fixture? How do you have it connected exactly? The neutral for the light goes to the load-side neutral terminal on the GFCI, not to the line side, right?
Have you confirmed that there isn't actually a ground fault? The GFCI may just be doing it's job. A hot wire in contact with plaster or masonry may leak enough current to trip a GFCI. A hot wire touching metal lath certainly will.
Have you confirmed that there isn't actually a ground fault? The GFCI may just be doing it's job. A hot wire in contact with plaster or masonry may leak enough current to trip a GFCI. A hot wire touching metal lath certainly will.