What downloader software or downloader add-on did you install?

https://www.dvdvideosoft.com/products/dvd/Free-YouTube-Download.htm Use this YouTube downloader to get single videos and complete playlists, show list, charts and user channels from YouTube. Convert to AVI, MP3, WMV.

Hi   !

Separate issue :

You need to update Flash, for your system details show a really old version, which can cause problems.

The latest version of Flash is 26.0.0.137

You can download a Flash installer here :

https://get.adobe.com/flashplayer/

Make sure to uncheck any unwanted extras that Adobe might be offering to include in the download.