Leak near a quarter-turn valve (undersink plumbing)

Hi,
This is a pipe in an old house, underneath a sink. Where the inlet pipe meets the quarter-turn angle valve there has been a small leak

I suspect this valve can be as original as the house (1970s?); the joint between pipe and quarter-turn valve is leaking slowly. I tried to...
Leak near a quarter-turn valve (undersink plumbing)