Cryptocurrency investor Justin Sun, who paid a record $ million in a charity auction to have lunch with Warren Buffett, abruptly postponed his meeting with the billionaire investor Tuesday.

A foundation backed by the Chinese citizen tweeted that Mr. Sun had fallen ill with kidney stones, and was putting off a July 25 lunch with Mr. Buffett in San Francisco. It said both parties have agreed to reschedule the meeting.

A short time later, Mr. Sun denied a report by a Chinese media outlet that accused him of engaging in illegal activities within China. He wrote on his Weibo social-media account that rumors of illegal fundraising circulating on the internet weren’t true, and said his firm Tron wasn’t engaged in money laundering and other illicit behavior.

The sudden cancellation prompted broader questions on Twitter and cryptocurrency media outlets about Mr. Sun and his whereabouts.

Later on Tuesday, he appeared on the video platform Periscope, broadcasting from what appeared to be an office in San Francisco. The city’s Bay Bridge looks to be behind him.