(CNN)Liu, a 39-year-old tech worker in Beijing, arrived in the central city of Zhengzhou on Sunday with all the boxes ticked to travel under China's stringent Covid restrictions.
He had tested negative for Covid-19 the day before; his hotel had confirmed he could be checked in; and the health code on his phone app was green -- meaning he had not been exposed to people or places deemed risks and was therefore free to travel.
But when Liu scanned a local QR code to exit the Zhengzhou train station, his health code came back red -- a nightmare for any traveler in China, where freedom of movement is strictly dictated by a color-code system imposed by the government to control the spread of the virus.
Anyone with a red code -- usually assigned to people infected with Covid or deemed by authorities to be at high risk of infection -- immediately becomes persona non grata. They are banned from all public venues and transport, and are often subject to weeks of government quarantine.
That all but derailed plans for Liu, who had come to Zhengzhou, the provincial capital of Henan province, to seek redress from a bank that has frozen his deposits. He had put his life savings -- totaling about 6 million yuan ($890,000) -- into a rural bank in Henan, and since April hasn't been able to withdraw a penny.
Over the past two months, thousands of depositors like Liu have been fighting to recover their savings from at least four rural banks in Henan -- in a case that involves billions of dollars. In late May, hundreds of them traveled to Zhengzhou from across China and staged a protest outside the office of the Henan banking regulator to demand their money back -- to no avail.
Another protest was planned for Monday. But as the depositors arrived in Zhengzhou, they were stunned to find that their health codes -- which were green upon departure -- had turned red, according to six who spoke with CNN and social media posts.
Dozens of depositors were taken into a quarantine hotel guarded by police and local officials, before being sent away on trains bound for their hometowns the next day; others were "quarantined" at several other locations in the city, including a college campus, according to the witnesses and online posts.
Depositors accused Zhengzhou authorities of tampering with the health code system to prevent them from returning to the city -- and thus thwarting their plans to fend for their rights.
"The health code should have been used to prevent the spread of the pandemic, but now it has deviated from its original role and become something like a good citizen certificate," said Qiu, a depositor in eastern Jiangsu province.
Qiu, a teacher, had not been to Henan to protest, but his health code also turned red on Sunday evening after he scanned a QR code from Zhengzhou. He said a fellow depositor had shared a photo of the Zhengzhou QR code on the WeChat messaging app, in an attempt to find out whether depositors outside Henan were also affected.
The red code seems to target only depositors. Qiu used his wife's phone to scan the QR code, and it came back green, he said. "I called the government hotline in Zhengzhou to complain about my red code, and they told me there was some error with the Big Data information database."
Liu and Qiu both asked to be identified only by their surnames.
CNN has reached out to the Zhengzhou government for comment. The Henan Provincial Health Commission told state-run news website thepaper.cn it was "investigating and verifying" the complaints from depositors who received red codes.