I am finding it hard to diagnose. It isn't serious, YET! Because the leaves are quite chubby they resemble how my earlier grows looked. I had a 22L NFT hydro in a confined space with high temperatures in the 30s. The plants were transpiring lots of water and the grow space was like 70%+ humid. This gave us quite wide fingers on the leaves. Because of your high temperatures outside it might not be possible to reduce your grow room daylight temp to 25C. Bringing down the temp should bring down the humidity, a small closet extractor would be ideal for sending the stale air some place else. See how it goes and keep us posted.

Oh. If you can get your fan pointing on your plants causing the plant to move, this would help strengthen the stalk and branches early on and also move air/oxygen from around the leaves. What is the N-P-K ratio of your nutrients (it should be somewhere on the label)? What is the content of your enriched soil? How big are your pots? How long have they been vegging? When did you start to feed and are you using full dose of nutrient solution according to the guide on the label?