Just my opinion, but one feeder for 20 birds is not enough. A very low pecking order bird is going to have trouble getting adequate feed access. Several small feeders spaced around would be better so that lower birds can access one away from any higher birds that will keep them away. Same with water. I have 6 feeders for 28 birds.
Without a necropsy there is no way to be sure what happened to this particular bird. Could have been internal parasites, they are not always visible in droppings. Could have been a blockage somewhere either from parasites or eating a foreign object that did not pass. Could have been disease. I'm very sorry for your loss.