Athosanswered grow question 7 years ago It's definitely nutrient burn. Ever heard or read the expression "burnt to a crisp", just like your "crispy leaves". Your nutrient solution is really high for the veg phase, dial it back to 1.0EC/500PPM, then when you switch to flower ramp it up slowly up to 1.4EC/700PPM.
The low humidity might be causing your plants to drink more water, forcing them to take more nutrients than it's safe for them, if you can't rise your humidity, use a lower EC/PPM than normal