If you are happy to get into the shell command line, through Webshell, telnet or SSH, you can find the ffmpeg process and kill
it (pkill ffmpeg
). That will cause the job to fail.
Otherwise, restarting the box will definitely stop job 21 and cause it to restart (there are.more extreme solutions but I assume you want to carry on using the box!). You may be able to Hold the job before restarting the box (that won't affect the running ffmpeg, I think), or you may be able to Hold or Delete the job in the queue manager before it restarts after rebooting.
The queue manager doesn't have an Abort job function, as discussed elsewhere.
When the queue manager sees a failed job, it's queued for restart (up to some retry limit, possibly 1, after which it can be Re-submitted). On restarting the problem job, youtube_dl will check from the original URL what files it should download, see that the intermediate files have already been downloaded, and then restart the problem fix-up process.
The qtube.log entries will show what's actually happening under the hood.