Hi Sebastiaan Weyler,
It could network issue, sometimes it happens..
You can try to change your network, and it shall work.
Hope it helps!
Small update...
I removed all Mendix software from my computer. Installed version 10 (instead of 11).
This time i did not click on "Create App" from the web-portal, but I clicked on "Create app" directly in Studio Pro 10, and then I had no issues at all.
So for now I can continue my course, and I will avoid starting new apps from the portal... but still not sure what the core issue was.