These probably a very subtle difference somewhere and maybe another poster will explain it, but in my opinion both really mean the same thing and you can use either. I suppose we’d usually use grasp if you can’t actually understand the problem, and get to grips is more often used when you actually don’t want to or don’t make any effort to understand the problem. But it’s a very tiny difference in meaning.

In my opinion, both of these imply that you know the cause of the problem, you just can't understand what to do about it, etc. If you truly don't know the cause of the problem, it should be something like: I don't know, I can't figure out, etc. the cause of this problem