Well there are different forms of dualism so this argument refers to only the one formulated by Descartes. The problem is this: if you posit that the physical and the mental are completely different in kind - that they are two completely different substances - then how do you have interaction between the mental and physical? How can the mental causally affect the physical, ie when I will my arm to type, how does that happen? Where does the causal interaction take place?
Descartes could not find an answer to this problem. Famously, he first said that all interactions between the mental and the physical take place in the pineal gland which is a ridiculous answer as I'm sure you can appreciate. Once he realised that this answer just pushed the problem back further, he said that the interaction takes places through a mediation of God or in other words, God provides an accidental interaction between the physical and the mental. Of course, that's just the first ever god-of-the-gaps argument.
The reason why this argument is important is that causation cannot be lawfully described unless there is complete knowledge of the space-time position of the interaction. For example, if my arms moves then unless I can conclusively prove where the cause of that movement came from, there is no way I can prove that I was the cause. It could have been a mental entity outside of me for all I know, what possible differentiation could I have between two mental entities to determine who caused what unless I could give them a space-time location? Ofcourse, you can avoid this entire problem altogether if you state that all causation is every single time a miracle in itself and there are no laws governing it at all but I don't really think that is a viable option.
Regarding the "interaction" part, just like the soul, it's part of the supernatural... Yes, I believe in the supernatural. And by the way, I don't think there exists any logical solution to the hard problem of conscienceness.
Topic: I'm officially an Ex-Muslimah :)