Seo Jin-ho, a travel-agency operator in South Korea, wasn’t interested in exotic investments when a colleague first introduced him to PlusToken, a platform that traded bitcoin and other cryptocurrencies. But the colleague was persistent.

“You won’t regret this,” she said, according to Mr. Seo. She visited him several times early in 2019, telling him he could earn 10% a month. Finally, his skepticism gave way, and he bought $860 of cryptocurrency on the PlusToken platform.

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