The Tax and Customs Administration limits the number of people who can file a tax return at the same time.
The tax authorities have been struggling with a malfunction since Monday morning, which has still not been resolved.
Due to the malfunction, it is in some cases not possible to file a report.
According to a spokesperson, the Tax and Customs Administration is now on the right track to solve the problems, but the restriction is necessary "to keep the ICT landscape stable".
"Employees work day and night on a solution. Last night and last night, work was carried out on the IT infrastructure to solve the problems as quickly as possible," said the spokesman.
In order to allow people to file a report successfully, the number of users who can be logged in at the same time is currently limited.
The Tax and Customs Administration advises you to try to log in again at a later time if that does not work immediately.
The maximum number of users is gradually increased later in the day "if the IT infrastructure remains stable".
The cause of the malfunction is still being investigated.
On Tuesday, the spokesperson told NU.nl that there was no question of overload due to crowds or a targeted attempt to take the site offline with many login attempts, a so-called ddos attack.
Despite the problems, more than 350,000 people have successfully filed a report by 7 a.m. on Wednesday morning.
Entrepreneurs and private individuals have until 1 May to file the declaration for 2020.
Anyone who applies for a postponement before 1 May has until 1 September to file a declaration.