Washington’s state’s healthcare exchange shut down after the first few hours of open enrollment on Saturday, as state officials and software engineers tried to resolve a problem with tax credit calculations.

The much-maligned federal site, Healthcare.gov, appeared meanwhile to be working smoothly on the first day of the second open enrollment period for people seeking insurance plans under President Barack Obama’s signature – and singularly troubled – reform.

Officials at the exchange said Washington Healthplanfinder appeared to be working fine at first. When the exchange’s automatic quality control system reported the problem, they decided to shut the whole system down at about 10.30am, to fix it.

A message at the site said: “Washington Healthplanfinder is temporarily unavailable. We apologise for the inconvenience. Please check back later.”

The tax credits were off by just a few dollars in some cases, exchange chief executive Richard Onizuka said. He said the system would remain down until it can give consumers who want to buy health insurance accurate information.

Spokeswoman Bethany Frey suggested consumers try again on another day.

Open enrollment for healthcare insurance continues through 15 February, and officials are hoping as many as 85,000 people sign up. They also hope all of the about 145,000 people who bought insurance during the first open enrollment period, which began on 1 October 2013 will renew for another year.

Those who run the exchange had been hoping their computer system would handle traffic better than it did last year, when it closed down and rejected applications for reasons like a hyphen in a last name. About a thousand people who bought insurance the first time around are still having problems getting their payments credited and that money transferred to their insurance companies.

People who do not buy insurance will have to pay a fine when they file their income taxes. Those fines start at $95 or 1% of 2014 household income, but the minimum fine in 2015 will be $325 per uninsured person or 2% of household income.

Consumers will find more choices this time around, with more insurance plans and more companies on the state’s exchange. Rates have gone up slightly overall but some people will find cheaper insurance.