Good day Mees!
I have just looked into it and for as far as I know you can still sign up for the digital discovery day on 7th April via the first link below and the open days on the 8th and 9th via the second one. It at least works for me.
https://www.saxion.edu/studying-in-the-netherlands/open-days/digital-discovery-days
https://forms.summit.nl/Saxion/OnlineOpendagen/b66ab4d7-b2d7-4976-aca4-4caebb5f3887/started?_gl=1 *1ghlqh6*_ga*MTUyMTYwODUxNy4xNTczMTI4Njc1*_ga_XJCCT75JTR*MTYxNjIyNzg1NC4xMC4xLjE2MTYyMjgwMjkuMA..&_ga=2.69109457.1500603476.1616227855-1521608517.1573128675#/Page/0
It might just have been a technical error. Please feel free to let me know if registration works now. If not, we'll figure it out!