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I have tried to get the YouTube add-on (http://vaadin.com/directory#addon/35) to work, but for some reason it doesn't. The online demo is not working for me either. Is it only me, or is it a common problem.
Thanks for the report. I'm not sure what was causing this, but I made some changes to the player initialization and everything seems to work now - for me at least. I submitted a new version 0.9.2 to the Vaadin Directory: vaadin.com/directory#addon/35. Give it a try.