It's idiomatic, just a construction. Here are examples: "В чём разница?" (= What's the difference?) ; "В чём смысл?" (= What's the point?). And also there's is an idiom "В чём соль?" ('соль' - salt), which means ''What's the meaning? What's the main part of it?" (You can use it when you want to talk about some big question as religion or you just don't understand a joke, it can me used very widely)
This feels similar to the Dutch "waar zit het probleem in?" ("in what [aspect] is the problem located", something along those lines).
If that interpretation is right, it's actually only the latest in an odd number of parallels between Russian and Dutch so far, actually. (native English & Dutch speaker here).
There are quite some ship-related words as well, probably due to The Netherland's dominating influences throughout the sea in the past.
For example (as Gwenci stated - only in a nautical context): норд/noord, ост/oost, зюйд/zuid, вест/west, матрос/matroos, флагшток/vlaggenstok, рейс/reis (and many more).
Russian Peter The Great has been in the Netherlands. He learned new Dutch words there. :)
As a native Dutch speaker I must say that that sounds kinda off, slightly. It can be used for sure, but it's very specific. It's more for probing deeper. Lots of emphasis on the fact you want to narrow it down. I wouldn't use it often at all.
"I see you cannot finish your task. What exactly is the problem?" "Ik zie dat je je taak niet afkrijgt. Waar zit het probleem in?"
For just regular "What's the problem?" I'd translate it as: "Wat is het probleem?" "Wat is er aan de hand?"
Btw, I think it's really interesting how similar languages are, and how they relate. For example I am learning both Hindi and Russian at the moment and I notice quite some similarities between the two that definitely do not exist between either one of those and English, French, German or Dutch.
"Что это проблема?" is simply not quite Russian. "Что (это) за проблема?" is grammatically correct, but not idiomatic.
The reason why your version is wrong is because Russian "Что" is more restrictive than English "What is...?" - by itself it cannot be used to ask for a description of something, only about identifying the object: Что это? = What is it? The answer should be the name of the thing, not its description.
That I understood. I mean, why would you use в чём instead of Что? Why is Тим, Что проблема wrong?
It does not work that way, you cannot just say 'что проблема'. Literally this example here means 'in what is the problem'. Despite being a native speaker I am not sure if I can tell you why we think that the problen is not a certain object or action, but inside some situation, it's more like 'where' or 'what does it consist of' than just 'what' -- you just have to keep that in mind. The answer may be: Проблема в том, что ты дурак (the problem is [in the fact] that you are a fool); Проблема в тебе (the problem is [in] you)