This week I didn't spend much time checking in on the tent, but when I went to check the reservoir, which I expected to be empty, I realized that the system was clogged. About half of the nutrient solution was still in the tank.
I opened the tent to check the moisture of the substrate. It was pretty dry already and the plants were showing some heat stress and the burnt tips had gotten somewhat worse. Being too close to the light didn't help either. They weren't hanging yet, but they needed to drink.
It was clogged in a small part of piping between the tank and the tap, which I realized, was never actually cleaned before. And I did have some organic compounds in there a rather long time ago, which probably caused this in the first place. I unclogged it within a few minutes, and bought some Canna D Block to try out. I will probably refill the reservoir again early next week.