Same here as well.... But you can use Siri from the home screen by holding the Siri button and saying "Search Youtube for 'insert search here' " and it will launch Youtube and do the search. In fact you can also use Siri from within the youtube app by asking it to do a Youtube search for whatever.
Siri works fine in the YouTube app, it's dictation in the search box that does not work...
Try the following, from the main YouTube home page (do not select the search box) hold down the Siri button on the remote and say "Search YouTube for 'what ever' " and it will do the search and take you to the results:-