Hi Unkraut, sorry but i do think it is typical N overdose, your first clue is that it started during flowering when the required NPK profile starts shifting away from N. The other symptoms include starting to see burnt tips , the "too green" aspect of some of the leaves and the shiny aspect of some of the leaves. It looks like using all those additives are adding up N to the plants up until the point where it's now just a bit too much. All kelp extracts have N in it like alg a mic and rhizotonic, and i havent checked every label but i'm quite sure you dont need so many additives, even though they probably have helped these plants control root problems. So take some N out to get a better adapted NPK ratio and the burnt tips will stop spreading. Hope this helps ! 🚀