I asked other people as well and they don't have this problem. Also, when I select 'OpenStreetmap German Style' it works fine. I noticed that the error has changed from 418 to 403 now (Failed to load resource: the server responded with a status of 403 ()).
When I open one of the tiles (for example 'https://a.tile.openstreetmap.org/14/8447/5371.png') of the map in the same tab of the browser, it also loads fine, isn't that strange in case I would have been cut off?
Next to this, when displaying the map in another browser (Edge), the 'OpenStreetMap default"-map is also shown without any problems.