Are you using a or a Multihoster (like Real-Debrid)? Does the link work if you try it directly in your browser ?
Large platforms like YouTube frequently update their backend APIs specifically to break external downloaders. These "cat-and-mouse" updates can cause JDownloader to grab a link that YouTube immediately invalidates. Are you using a or a Multihoster (like Real-Debrid)
Some link protectors (e.g., sh.st, adf.ly) present an intermediate page. JDownloader decrypts the first layer but fails on the second. The final URL points back to the same decrypter (infinite loop) or to a blank page. Are you using a or a Multihoster (like Real-Debrid)