Link works for me (firefox 125), but only if I use http (port 80), port 443 get me 404 not found as well.
Just tested, works fine with Safari (mobile, but desktop usually has the same behavior).
Chrome's attempt to use a secure connection resulting in 404 while the actual link works fine isn't great. But it's understandable and the TLS-first assumption the're using probably works just fine in the other 99.999% situations.
If only they'd taken the time to copy the http config and setup let's encrypt or cert from GEANT Vereniging CA apparently preferred by rug.nl...