Because file hosts update their security daily, keeping JDownloader updated is your first line of defense.

Instead of the video/file I was expecting, JDownloader attempted to save an .html file (which is usually a webpage) rather than the actual media.

Forcing JDownloader to fetch a clean connection state often clears temporary routing glitches. Right-click the broken download task.

If you are technically inclined, you can inspect the actual final URL that JDownloader is trying to use.