What is the problem?
¿Cuál es el problema?
Why isn"t it ¿Qué es el problema? Because it is interrogative?
But ¿qué es eso? is interrogative..
"Cual" literally means which in English. I think of it as there can be many problems but which is yours. With ¿qué es eso? there's only one definitive thing it's inquiring about. On the other hand, you ask ¿Cuál es tu nombre? Because there are many names you could have.
Que es tu problema?