This error occurs at those times when you change your SIM and the default internet settings in your applications changes by itself.
Go to Real Player>Settings>Connection and change the default setting to the one you wish.
And also, change the default access point in your default browser(s) to the same setting you put in Real Player.
Then open the Youtube website using either default browser or Opera Mini or any other browser. Clicking on any video there will open the Real Player and the video will play automatically.