Update 2021.
As usual This is a great app, and as usual, the problem, now sorted was caused by google. No surprise there though.
Without a doubt Easy Video Downloader Express is the best Youtube downloader, without exception. Since having the pro version, the only hiccup was caused by Firefox and it's continual making alterations. Make sure the one you choose has EXPRESS, the other one doesn't work. Read their infomation, to find out more