It looks like you included the YouTube video at that specific width.
You can be better off choosing a larger width while grabbing the embed code, or just paste the URL to the YouTube video into the editor and let WordPress/GP figure out the best width.
Hi Tom
Thank you for your reply.
No, this is something else. The pure URL are just copy/past without any other info.
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Found! This comes from JetPack. Once deactivated, things run well.
I used it for a little CSS… I’ll use another solution than this one