T
Toneyz
under cabinet lights must be off a different circuit - plugged into sockets.
There must be a short between that red and neutral/earth somewhere.
Have you done any other work that may have caused it?
Screwed into a wall and damaged a cable?
A professional with a test kit will be able to find it.
I think we are just guessing here. The O.P. knows from his photo that the red and black was not across the switch. There wasn't this problem before.I bet when he’s connecting 5th cable he’s wiring l and n across the switch.