It seems to me that they are creating a file on you so that they can fire you. Your boss is never going to answer that question truthfully. The file will "show" that you were incompetent at your job and so when they fire you they can say that it was not because of your depression. That is how I see the situation. Your company does not want to be held liable for discriminating against you so they come up with other reasons to fire you. I would also talk to a lawyer. Often law schools have students do free consultations. You might want to look into that if there is a law school near you. Good Luck.