YouTube video streams can be broken. The program can't download such streams and you get the error 'Couldn't download HTML page (105404)'.
You should choose a different video stream available for downloading:
- add the failed YouTube link to the program window one more time
- click on the blue link under the video title and take the best available video stream from the drop-down list (see the screenshot below).
Note: if you don't see the link und get the error message as soon as you add the YouTube URL, you need to deactivate the auto-download mode. Open Tools - Option and uncheck the box for Auto-download.