What version of Muse?
Back in Muse 7 a change to the video player used by Google for YouTube videos would result in the behavior you describe. In Muse 7 that was worked around by using the YouTube widget from the Widget Library provided with Muse 7.3.
Muse 2014 (the equivalent of version 8) replaced the HTML rendering engine used for Preview, and generation of proxy images for Design view. That improves the fidelity of Muse Preview and the proxy images, and avoids this crash/hang when inserting YouTube embed code.