i discovered now that only https://feddit.nl/c/nieuws is browseable through this server, but no other communities. I'm confused lol
You get a 404 because the other communities haven't been added to your instance yet. To make communities and posts from other instances visible, someone has to manually add them first. You can do this by heading over to feddit.nl, taking the link of any community you wish to add, and then pasting that link into the search bar of your home instance. At that point the community should finally appear and you can subscribe to it. Hope this helped!
Notably, it can take a minute or two sometimes. It's a bit of a process to get groups federating the first time. I'm hoping that improving group discovery is prioritised by the devs some time soon
Keep in mind, when you say "this server" that doesn't mean anything. I'm on kbin, but you're on blahaj.zone