WASHINGTON — Walt Whitlow was under treatment for cancer when he got an unwelcome surprise.

His financial assistance under President Barack Obama’s health care law got slashed. That meant his premium quadrupled and his deductible went from $900 to $4,600.

Thousands of people lose subsidies under the health law, or even their policies, when they get tangled in a web of paperwork problems involving income, citizenship and taxes. Some are dealing with serious illnesses like cancer. Advocates fear the problems, if left unresolved, could undermine the nation’s historic gains in health insurance.

Ana Granado was scheduled for reconstruction after breast cancer surgery when she was notified that her coverage would be canceled because of questions about her immigration status. Legal aid attorneys got that cleared up quickly, but Granado’s financial assistance for her premiums was suspended.

Lynn Herrin got irritated when she had to pay $700 to the IRS after it determined she got too big a tax credit for premiums under the health law.

Since she was also having trouble finding a doctor who accepted her insurance, Herrin canceled. It was a costly mistake. She was diagnosed with oral and neck cancer and depleted her family savings to pay for treatment.

Not every case is as distressing, but coverage disruptions due to complex paperwork requirements seem commonplace in the health law’s system of subsidized private insurance, which currently covers about 12.7 million people.

The government says about 470,000 people had coverage terminated through Sept. 30 last year because of unresolved documentation issues involving citizenship and immigration. During the same time, more than 1 million households had their financial assistance “adjusted” because of income discrepancies. Advocates say “adjusted” usually means the subsidies get eliminated.

“When people get that bill for a full-price plan, they panic and they cancel the insurance,” said Elizabeth Colvin of Foundation Communities, an Austin nonprofit that serves low-income people. Some worry the problem could undermine the law’s insurance markets.

“These problems can grow, and they can contribute to undermining consumer faith in the system, and that could lead to attrition,” said Rachel Klein of Families USA, an advocacy group that supports the health law.

Pressured by a Republican-led Congress hostile to Obama’s overhaul, the administration is highly sensitive to criticism that some people may be getting benefits they’re not legally entitled to. But Health and Human Services Secretary Sylvia Burwell recently acknowledged the paperwork tangle is more likely to trap the innocent than fraudsters. The administration has made customer retention a priority, and Burwell said she’s focused on making the system less challenging for consumers.

The next few months will tell whether improvements to the HealthCare.gov website in the 2016 sign-up season helped cut down on problems. The technology now alerts consumers to gaps in their applications that could later trigger documentation problems.

If there’s a documentation issue, consumers have about three months to straighten things out.