Go to Settings > Apps > YouTube > Clear Data/Cache.
After this version, Google pushed an update that changed the underlying video player framework (ExoPlayer v2) which is incompatible with Lollipop’s WebView and media codecs. youtube android 5 version full
If you don't want to install new apps, using a mobile browser is the official workaround recommended by Google. : Open your browser (Chrome or Firefox) and go to m.youtube.com : For a better experience, use uBlock Origin Go to Settings > Apps > YouTube > Clear Data/Cache