An attempt to fix a glitchy data system forced Sun Country Airlines to cancel flights for a second day Tuesday, and angry passengers at Minneapolis-St. Paul International Airport overwhelmed its customer service operation at one of the busiest times for travel this year.

The Minneapolis-based airline announced a "pause" to its operations shortly before 1 p.m. so that a technology supplier could install software to fix a problem that produced cancellations and delays Monday.

The two days of disrupted service exposed weaknesses in the way Sun Country books tickets and responds to customers. For Cory Williams of Woodbury, it all meant that a trip he and six other members of his family planned in Arizona this week, ending at the Grand Canyon for New Year's, was off.

Among Sun Country's cancellations Monday was a flight to Phoenix that Williams originally booked for his family. He rebooked them on a Sun Country flight to Tucson on Tuesday, only to see that one canceled, too.

"I thought at this point, forget it," Williams said Tuesday afternoon. "To change hotel rooms, change rental cars again, it's just too hard."

Disruptions have frustrated passengers of many airlines in recent weeks. The rapid spread of the latest COVID-19 variant has sickened airline crews and staff, leading to thousands of daily flight cancellations over Christmas weekend, though just 1,200 Tuesday, according to FlightAware, a flight information service provider.

For Sun Country, the trouble this week sets back efforts to rebuild trust after customer-service debacles in 2018 and 2019.

Sun Country canceled all its flights before 8 a.m. Monday, citing a network system outage at a technology vendor. Those cancellations led to delays in its system throughout the day and at least one more cancellation Monday night.

With planes and crews reset, Sun Country began Tuesday normally. But by late morning, its vendor notified the company of the plan to upgrade its software immediately. Unable to operate without that system, Sun Country texted and e-mailed passengers on flights that it needed to cancel in the afternoon.

But passengers complained on social media that Sun Country doesn't automatically rebook them onto other flights, a common practice at other airlines. Instead, it asked passengers to buy tickets for new flights and then seek refunds for the canceled flights.

"I have 14 tickets needing a refund," Williams said. "I'm not going to try to rework anything. I just want our money back."

On its website, Sun Country said Tuesday's system update required it to cancel "seven out and back departures at our Minneapolis-St. Paul hub." The airline added: "We recognize that this news is unwelcome and disruptive to our passengers and their loved ones and apologize sincerely for the necessity of this action."

On July 1, Sun Country canceled four of 91 scheduled flights, saying crew-scheduling software provided by Dubai-based AIMS International went down. It manually cleared crews for takeoff, something the airline said it also did during Monday's outage. Sun Country has not publicly identified the vendor responsible for this week's problems, however.

Sun Country in 2019 turned to Minneapolis-based Navitaire to create the main infrastructure used for its reservations, website, travel agent portals and the processing and boarding of passengers at airports.

This week's service disruption affected far more passengers than a headline-making episode in April 2018, when a late-winter storm forced Sun Country to cancel flights on routes from Mexico to the U.S. Timing worked against the airline because those flights were the last of its winter season service to Mexican resort cities. The cancellations stranded 250 customers, who had to find their way home on other airlines.

Anastasia Salazar, a graphic designer from San Francisco who visited the Twin Cities for Christmas, arrived with her partner Monday night at MSP for their Sun Country flight home. But when they reached their gate, they found it was canceled.

"They canceled it about 40 minutes before takeoff," Salazar said. "There was no explanation, and they didn't offer any type of hotel or refund."

She said an agent told her to return to the airport Wednesday morning for a new trip to San Francisco, and the couple found a local Airbnb owner who took a last-minute reservation.

On Tuesday morning, Salazar waited on hold for two hours on the airline's customer service hotline to get more details. After speaking for a few minutes with an agent, Salazar was told the airline couldn't offer her anything. When she pressed, Salazar said, the agent told her, "I've been advised by my supervisor to hang up on you."

Early Monday, Anna Henderson of St. Paul, along with her husband and son, were driving from a beach to the airport in Fort Myers, Fla., for their afternoon flight home on Sun Country, when they saw an e-mail that the flight was canceled. An airline spokeswoman said that flight was canceled for reasons unrelated to the system outage.

The family encountered helpful agents at the Fort Myers airport who rebooked them for a 9:15 p.m. flight, They eventually departed at 10:30 p.m. and arrived at MSP in the wee hours of Tuesday morning. It was the family's first vacation since the coronavirus outbreak, and Henderson said it showed travel in the U.S. is not back to pre-pandemic normal.

"We made it there, had a great time, but it felt like a lot of gambles," Henderson said.

She said she kept asking herself two questions: "Should we have just stayed home because the world is not working the way it used to? Or do we believe there's a social compact and we can do these kind of normal things and it's OK?"