Reload of the RDF file fails with
<body><h2>HTTP ERROR 404</h2> <p>Problem accessing //bigdata/namespace/dcatap20210827/sparql. Reason: <pre> Not Found</pre></p><hr><a href="http://eclipse.org/jetty">Powered by Jetty:// 9.4.12.v20180830</a><hr/>
The attached patch should fix it.
https://dcatap.wmflabs.org/ on the other hand is no longer available and I'm not sure that it's worthwhile to set it up again. It probably was a WDQS GUI targeting the DCATAP dataset, I'll update the doc to frame it like the categories dataset pointing users to the sparql endpoint https://query.wikidata.org/bigdata/namespace/dcatap/sparql.