You can ask questions via data.overheid.nl via the response form from the KOOP service desk (link to external website). On the form, select the option 'User of a KOOP application or service'. Under 'I have a question about', select the 'Open Data' option.
Visiting address
Kennis- en Exploitatiecentrum Officiële Overheidspublicaties (KOOP)
Rijkskantoor Beatrixpark
Wilhelmina van Pruisenweg 52
Building D (Orange), room 1.001
2595 AN Den Haag
Please bring a passport or EU identitycard (or Dutch driver's license) to be able to enter the building.