BEIJING — Brilliantly simple, emission-free solution to the problem of urban congestion? Or the maddening height of venture-backed technology hubris?

Dockless rented bicycles swept into Chinese cities two years ago. Their arrival inspired a wave of similar mobility experiments around the world, even as city dwellers in China bemoaned the sight of candy-colored bikes heaped in unholy piles on their sidewalks.

Now, one of China’s leading bike start-ups, Ofo, is facing serious financial problems. Its founder is on a government blacklist for unpaid bills. Millions of riders who placed deposits are demanding their money back. And the business model used by many of China’s tech firms — spend furiously to acquire new users, worry about profits later — is showing its limits.

The run of Ofo customers seeking refunds appears to have started last week. Doubts about the company’s financial health had swirled for months. People said on social media they were unable to get refunds on deposits of $15 to $30, which they put down to rent Ofo’s bumblebee-yellow bikes. (The company’s Chinese nickname translates as “Little Yellow Bike.”)