Bank blocks account after buying BTC

Today I received a letter from my bank noting that my account has been blocked due to money laundering and terrorism financing laws. This came just a few hours after purchasing bitcoin on cex.io. This is not the first time i bough btc there, but the total amount is rather small, about three hundred usd.

Now I will have to go and talk to them, and they are asking for further personal documents.

What strategy would be the best to use here? Thanks for the advice.

Edit: The bank is UniCredit and I cannot cancel it because my country pays out stipends only via this bank. Talk about monopoly.