A cyberattack hampered the launch of the Netherlands' COVID-19 passport.
As a result, it was at times difficult to download the required QR code late on Saturday, the Ministry of Health in The Hague said on Sunday.
The attacks, which blocked server capacities, had been averted.
Holders of the passport can prove they have full vaccination protection against COVID-19, or that they have recovered from the disease. It can also provide proof of a negative test.
Since Saturday, the certificate has to be shown by people aged 13 and older who want to get into restaurants or cultural or sports events in the Netherlands, but it is a controversial topic.
Prime Minister Mark Rutte dismissed State Secretary for Economic Affairs Mona Keijzer on Saturday following her criticism of the introduction of the COVID-19 passport in an interview with the newspaper De Telegraaf.
The criticism was not compatible with the government's course, Rutte said.
On Saturday, there were several demonstrations against the use of the passport.
After around 18 months, the Netherlands lifted social-distancing rules. There is no longer a restriction on the number of visitors at stadiums, pubs or shops when the COVID-19 passport is presented.
Those who are not vaccinated or have not recovered from COVID-19 can be tested free of charge. The use of masks remains compulsory on buses and trains.
Around 82% of adults are fully vaccinated in the Netherlands.