Loop at switch - 2 gang switch

[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 bet when he’s connecting 5th cable he’s wiring l and n across the switch.

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.

 

[Squid]

Did you have a Spirograph.

Etch a Sketch it seems.......