This won't end well.

You don't say what you mean by "some crypto profits". If you mean 500 EUR, I don't think no-one will care. But if we are in the 5 or 6 digits figure, and supposing your girlfriend's Belgian bank account does not receive monthly more than 4 digits figure, then that will raise a question for the tax office. They will wonder why your girlfriend's account has suddenly an increase of X amount and she will be in trouble.

The only way I see here is for you to transfer the money (as a "gift") to someone you trust in Serbia and who lives in Serbia. Then you need to see if this transfer ("gift") is tax exempt in Serbia for the receiver (gifts are taxable in many countries). Then the receiver can cash out, and then you can get the cash by going there in person. The other option could be opening an off-share bank account e.g. in Switzerland, but I don't have a clue how that works.
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Thank you for your reply.
My girlfriend does not have a Belgian bank account yet, so I suppose that no amount would be suspicious in the beginning.
Even when she already has some money on it, it could well be that she had way more on her Serbian account.

We are talking about 5 digits, what concerns me is that my girlfriend could be in trouble because the amount on her Serbian bank account would increase a lot out of nowhere.
Is that something I should worry about?

Than you.