Exact versions used of packages (related to UI):
Jeroen,Try to update system.web,system.web.http packages to latest version.
@Karyan, both are running on the latest version. Will check again with a non demo license as that license is also giving some trouble.
Found out that the issue was related to a Demo Index I've used. That service is not relyable to test. I tested it with a normal license and that one seems to work as expected in combination with latest versions of all packages.