Hi Tomuk,
first and foremost I am so sorry for the passing of your dog. Very tough to do and my last dog was 13 when she died in my arms. Now I have this beauty to my left and I know it's going to be just as tough when the time comes to make that decision again.
OK so you have gotten some great advice. I went through your journal and I don't see anything that resembles nute burn. What I do see is a plant that is struggling with nutrient uptake from an incorrect pH. Back has mentioned this and in coco you are running what is called passive hydro so there is nothing to buffer the nutes such as limestone which you would find in Promix or Fox Farm as an example.
The plants are light in color and do need more food. I can see why you are hesitant because more food means your pH will be lower which is bad for the plant. I think this is why you are relating the nutes to the burn when it's actually a pH issue.
If you are feeding by the 1.5l then the plants should be getting 1.6g/1.5L of water at this stage.
Anyhow I hope that helps a bit. Back has given you some good advice. Feed more and fix the pH issue.