The most frequent cause is an outdated plugin. File hosts constantly change their website code to prevent automated downloading, and JDownloader developers release daily updates to fix these "broken" plugins.
JDownloader relies on specific plugins to navigate the unique APIs and obfuscation methods of sites like YouTube or Rapidgator. Frequent website changes by hosts can break these plugins, requiring an official software update for a fix. Security and Geo-Blocking: jdownloader host problem