ok, firstly, flushing is a kind of last choice thing, nute lock doesn't just happen, there are symptoms, do you have a ph meter? when your ph gets above 7 in soil that is a sign that flushing is necessary, but a ph down formula may also fix this, so again flushing is still a less optimal choice, its also stressful, I'm not ruling that out as the cause for your leaf troubles, and yes browning and crisping are common in the flowering cycle as buds are basically sucking any mobile nutrients from nearby foliage. It may also be whatever you are feeding them, remember to use only a quarter to half strength of the recommended dose on the nutrients directions, unless the nutes are actually formulated for cannabis.
heat and humidity may be factors as well.
once a leaf gets all dead and yellow and crispy there is no corrective action but to remove it, do not however remove it if there is still green on it, that means it is still photosynthesizing.
if its because of anything else here is a handy set of pictures to have