The most frequent complaints are headache / migraine after ingestion of chocolate. These reactions are probably more intolerance or sensitivity than food allergies.
Chocolate is known to cause headaches, obesity, heartburn and irritability and depression. Most probably these reactions are due to other ingredients of chocolate such as milk, soybean, lecithin, gluten and nuts.
There are 380 known additives in chocolate. They can have pleasant and unpleasant effects.