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So, YT changed something yesterday which broke the change that XenForo had added in 2.2.6 to support playlists - which explains why it didn't stop working immediately after I applied Patch 1 some days ago.
Since the forum is using oEmbed, that probably means that when the forum makes an oEmbed call to YT to get the embed code for a video URL, it's being returned with the empty '&list=' URL param, which then breaks the embed.
That would be YT's fault - twice. Not only should a blank 'list' param not be returned if the vid URL is not for a playlist, but the fact that there IS a blank URL param in the embed code URL shouldn't break the embed, either. Bad programming!