We have resolved the Downloader issue.
Please follow these steps to have your Downloader work again.
1. Open Internet Explorer
2. Click Settings and open Internet Options
3. Click the Advanced tab and hit the Reset button
4. Close Internet Explorer
Once you are done resetting your Internet Explorer, please follow these steps:
1. Using Chrome, Firefox or Edge, go to YouTube.com and play any video and click the Downloader button. (This action will still show "Unable to download" message)
2. Close all Windows
3. Restart your PC
4. Go back to Youtube.com and try to download any video (This should activate the latest version of Downloader and you should be able to download videos.)