“When is it permissible to discuss others’ problem (if ever)?”
Recently I was asked to help collect funds for a neighborhood family, to help them make a chassuna. My Rav told me that I was forbidden to tell people who I was collecting for (they would be embarrassed), and the only situation where it was permitted, was if it would result in the person giving more if they knew who they were giving for (and even then, there were conditions – those situations I had the Rav make the call!).
I would say it is the rare circumstance where one is permitted to discuss others’ problems.