Actually I found the solution in another post : you have to add in your short code identify_by=”channelid”
So for you it would be (with your API key):
[Youtube_Channel_Gallery identify_by=”channelid” user=”UCMr6SIB91BZ2VXXVrsRcM2g” key=”your_Google_API_key” maxitems=”16″ thumb_columns_tablets=”4″ title=”1″]