Suddenly, massive problems. Tried a lot already, nothing helped yet. Any ideas what I'm missing?

Unfortunately it is the end to the growth. I suppose there was a huge overfeed of some nutrient or by general on plant I think that it is possible to be a Fusarium on the roots, by the general burn. I can see how many pots are you use, but that two things would possibly happen...or a massive overfeed, but I can't imagine that.you have done a massive induction of a nutrient without seeing the evolution and the burn coming or possibly the soil where you grow is contaminated of Fusarium. Other possibilities could happen if you have a hydroponic system. Normally what I do not use again were th etchings that you used on the grow such as soil, watering solution if is hydroponic and I would wash pretty much with bleach the pots and dry them on the sun for some days if the case was a fungus, due to the spores that can remain. It is really pitty to watch a whole grow on the trash bin. I'm sorry mate, but that grow is officially dead. It is a non recovery situation. A big hug and Jah God always with you, brother.