It has been more than seven months since the new U.S. Electronic System for Travel Authorization (ESTA) has been mandatory for airlines that fly citizens of visa-waiver countries to the United States. Yet some carriers’ computer systems are reportedly experiencing serious problems, resulting in denied boarding for travelers with valid ESTAs.

Last week, I received a disturbing e-mail message from an Austrian citizen who had read my previous coverage of ESTA issues. On July 5, she wasn’t allowed on a Scandinavian Airlines (SAS) flight from Stockholm to Chicago, for which the carrier blamed problems with the passenger’s ESTA.

In fact, the customer, who asked that her name not be used because she hasn’t yet resolved the matter with SAS, had a valid ESTA on which she had previously traveled to the United States. The validity was confirmed by my sources at the Department of Homeland Security (DHS), the federal agency in Washington that administers ESTA.

There were no signs of any problems when the passenger checked in at the SAS counter in Stockholm, according to her account. Her boarding pass, a copy of which she sent me, clearly shows “API OK,” which means that all necessary documentation was in order. However, when the boarding pass was scanned at the gate, it produced a sound indicating that not everything was actually in order.

She was then sent to a customer-service counter, where only one agent was equipped to handle ESTA issues, and she had to wait in line — with her flight already boarding. A German citizen in front of her in the queue had the same problem, but the agent was able to request a new ESTA for him that was instantly approved, and the SAS system was able to establish a link with the ESTA system and issue a boarding pass.

But the Austrian traveler had no such luck. No fewer than six new ESTA requests were approved as soon as the agent submitted them, but the SAS computer was never able to link them to the airline’s boarding system.

The passenger missed her flight and was told that no seats were available to the United States for the next two weeks. Her “dream vacation,” as she called it, which had taken weeks to plan, never happened. SAS refunded the miles she had used for the award ticket, as well as the money paid for taxes, but she claims she lost about $2,500 in non-refundable domestic U.S. airline tickets, and hotel, rental car and tour reservations.

SAS, which is a member of the global Star Alliance, has yet to respond to her letters, but that’s not what she’s most worried about.

“What makes me angry is that they are obviously having problems with passengers traveling on [non-Scandinavian] passports and have not bothered to do anything about it — not even a simple measure, such as asking these people to show up at the boarding gate a little earlier to settle things,” the Austrian passenger said.

I e-mailed SAS spokeswoman Elisabeth Manzi in Stockholm asking whether the carrier was aware of the problem and if it’s doing anything to fix it. Instead of responding, Manzi forwarded my message to Martina Vercellini, a customer-relations representative in Frankfurt, who wrote me that she would contact the Austrian traveler directly. It’s not clear why SAS hasn’t done so seven weeks after the incident.

Still, as the passenger suggested, the problem is bigger than her. “Now I’m wondering how to avoid similar problems in the future,” she said. “For my next trip to the U.S., I’m very much leaning towards getting a visa beforehand, whether it is necessary or not” — and despite the $140 visa application fee.

On Sept. 8, DHS will start charging ESTA applicants $14 — $10 for the new Congress-mandated U.S. “travel promotion” initiative, and $4 to “recover the costs incurred [for] providing and administering the ESTA system.” Perhaps DHS could make sure that airlines are capable of verifying the validity and authenticity of an ESTA, so fee-paying travelers aren’t denied boarding for no good reason.

It’s important for passengers to know that each ESTA is linked to the passport for which the ESTA was originally requested, so if you get a new passport, you also need to apply for a new ESTA. That wasn’t the case with the Austrian citizen — she has had the same and only passport since 2002.

My DHS sources also told me that tens of thousands of people are applying for an ESTA every day. If you want to beat the $14 fee, make sure you apply before Sept. 8 — an ESTA is valid for two years, so it makes sense to do it if you plan at least one U.S. visit in that period.

Many travelers consider ESTA a de facto visa — except that it’s much cheaper and faster to get than a regular visa, for which you have to be interviewed by a U.S. consular officer, wait in long lines and sometimes travel long distances from your hometown.

Currently, 36 countries participate in the visa-waiver program, but that number can change at any time, as their qualification is reviewed periodically. To be eligible for an ESTA, you must stay in the U.S. no longer than 90 days and possess a machine-readable passport. Other passport security features also apply, depending on the issuing country.

Just like with a visa stamped in your passport, holding a valid ESTA doesn’t guarantee you admission to the United States, which is at the discretion of the immigration officers at the point of entry.

However, if you have a valid ESTA linked to the passport you are traveling on, and no warning has been issued by the U.S. authorities against you, airlines should be able to verify that and let you on the flight you’ve paid for.

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U.S. visa-free travel comes with strings