Considering how much of that 0.75mm copper is showing I'd suggest loose termination on the GU10 lamp holders.
and it being loose on one would affect them all?
Have a look at the switch that controls them in that case.
@rogan started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
Considering how much of that 0.75mm copper is showing I'd suggest loose termination on the GU10 lamp holders.