I do not see the problem on the page http://prntscr.com/li9ri6 right now and you did not provide temporary wp-admin access to login.
Contact your hosting provider. Likely that problem is related to your server settings. Also, check the server error logs, probably you have some errors that could help to find out the reason.
Thanks for your quick response. That is really strange, when I go to http://cuijp.nl/wp-admin, I still get the 503 error…
But you checking that helped me find the issue here. I checked with some friends and they were also not getting the 503 error. I tried to go through the page while browsing using a VPN (hola) and that fixed it for me. I can now also access the wp-admin page.
It seems to have been some block based on IP address or mac-address, not sure.
But thanks for having a look, it gave me some clues!