Please follow these steps to resolve your Downloader issue.
1. Open Internet Options from the search window.
2. Under General Tab, click Settings in Browsing History and click View Files. (This will open a folder with a lot of files. Select all and delete them).
3. Close Internet Options by clicking Ok.
4. 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)
5. Close all Windows
6. Restart your PC
Try downloading a video and let me know if it helped.
If the issue persists, can you please let us know the following information:
1. Please go to C:\Program Files (x86)\Real\RealPlayer.
2. Right click on videodl and click Properties and then the Details tab.
Please provide us with the version number you see there.
Could you also provide us with a sample URL that you are trying to download?