This was a Slovenian crypto exchange.

The claim is that it was a "sophisticated hack".

Makes you wonder... what if someone wanted to open an exchange in a foreign country, only to have it get "hacked" for $64 million so that someone can claim "whoops, we got duped!", even though that someone is actually the duper.

I could be wrong, but because of how profitable BTC is right now, it would make perfect sense. There has been a lot of fraud with a number of exchanges as it is. I would bet my proposed theory is very possible and perhaps even likely.