what is the problem?

Could be several things, first thing that pops for me is iron defeciency, then magnesium or manganese. There's a great picture on Google by jorge cevanti I refer to. These signs will normally be towards the lower part of the plant. Usually caused by a nute lockout, if you've boosted the pk recently it could be down to that. Flush her well, increase calmag dose, reduce your pk dose slightly, and bring nitrogen down to a minimum. Luckily it doesn't look too bad, Jobs a gooden 👍🌱

Maybe flush through with half strength bloom feed until you get half the amount of run off as you have soil (so 20l pot you want 10l run off) If the new growth after a day or two comes out ok you can be pretty sure it was to much nutes causing block out. Also your leaves are curling up suggesting that your plants are struggling to cool down so maybe see if you can lower the temp's slightly 🤔 I hope she doesn't suffer too much and comes back stronger than ever 👍

Looks like a magnesium deficiency, A light green or yellow coloring will begin to show on the veins and edges of the lower & older leaves – this is one of the classic signs of cannabis magnesium deficiency. You may also see red stems. Magnesium is a mobile nutrient, which means that the plant can move it from old leaves to new leaves.