Can't load URL: The domain of this URL isn't included in the app domai

There's 2 separate issues you seem to be mentioning here. One is easy to correct, the other isn't as easy.

Can't load url: the domain of this url isn't included in the app's domains.

To fix this, please go to the Facebook Developer Area . Click into your app and go to Settings -> Basic. In there is a field for "App Domains". Add any domains you use for your site, which I would assume would just be:
glansbeton.be

Now, for the other issue in your quoted message from Facebook, it seems they have marked your app as a 'negative user experience'. That can happen for many reasons, including sharing content that others have considered offensive, 'fake news' type sites or anything else that may have caused users to report your site. In looking at your site, I don't see anything that would indicate this, so I'm not sure how you earned that negative view. You may want to submit an appeal and explain your sites purpose and how you interact on Facebook. It seems strange that your app was restricted from my perspective, but you know your site and content better than I do from a quick glance.

I hope the above helps,
Alex