Tips of new growth are burned
Not only upper growth, even on the lower branches...what could it be?
Lights are 40cm away
RH: 60%
Temp: 25°C
EC: 1.3
Watertemp: 18°
PH: 5.8-6.5
It looks like they are burning up from heat or light but from your grow information your temps look good.
I don't see any fans in there...do you have enough fans? Maybe you have hot spots from bad air circulation.