One thing I have found with using coco is that while it drains really well, it also holds water very well. I love it in a hydro setup but hate it in soilless setup. One key to coco is to mix it with a lot of pearlite, almost 50% will work just fine to keep it from compacting and holding moisture. I personally wouldn't use it in soilless myself as I have already tried and found the pH to be a nightmare to keep balanced. Also, I have found that coco holds nutrients better than it holds water.
In hydro, this is a benefit but in soilless it is a pain. To prevent buildup of both good and bad in the coco, you have to flush it periodically. This is why it works so well in hydro I guess as it gets continuously flushed and filled with fresh nutes for the plants to grab.
I agree with Kraven, don't use coco with FFOF, instead use pearlite to keep the soil loose and able to aerate well.
Here is a suggestion that may help you as it does look more like over watering. whenever you make up water/nute solution, aerate the water or solution for at least 12hrs with a fish tank bubbler so that it gets a lot of oxygen in it. Also make sure the water you use is no higher than 68-70f degrees as this is optimum for the plants likes, and it is optimum for max oxygen holding. The dissolved oxygen in the water will help the roots not suffer as much from excess moisture.