1. Temporarily disable your antivirus program (all modules).
2. Open DS3. Go to Help >> Manual Update of YT Downloader. Allow this to complete.
3. Close and then re open and now try downloading again.
If this doesn't solve the problem, then it is possible you have an older build and you may need to do a complete uninstall/reinstall and then follow up with this 3 steps again. If this also doesn't fix your problem, please complete an Incident Support Ticket here on the Help Desk