By Ada Petriczko

Over the four years that Krystian Wesolowski kept his savings on the cryptocurrency exchange Zondacrypto, nothing about the company raised alarm.

The 25-year-old social media manager from Wieliczka, in southern Poland, chose it for the same reasons that many others did: it was a familiar Polish brand, widely advertised and generally well regarded. The 40,000 zloty (9,435 euros) he invested had been steadily growing. One day, he hoped, it would help pay for the foundations of a house he was planning to build.

Then, in mid-April, a YouTube creator he followed mentioned growing problems with withdrawals. Wesolowski opened the app and found a notice: transfers were delayed, it said – a technical…