It is legal to transfer crypto profits from Serbia, from what I've read with their regulations, I don't think there will be a problem if you made a profit from trading cryptocurrency. Afterall, after transfering it to your girlfriend's bank it is already a fiat money. Serbia bank to Belgian bank transactions do not conflict. I believe fees are  not that too high or just a reasonable amount with Bank to Bank transfer.

You may take a look at this information about NBS (National Bank of Serbia) crypto regulations;

- https://www.globallegalinsights.com/practice-areas/blockchain-laws-and-regulations/serbia

Hope it may answer your question.