Apparently the information text found under a user’s profile link setup for YouTube is inaccurate (see first screenshot below). When someone adds only their channel “url” or “custom url name”, it unexpectedly directs to the BlenderArtists domain and not the expected YouTube domain.
Example: YouTube profile link
Result: When hovering over YouTube link (see lower-left corner)
One another besides me, and also Fweeb, who graciously suggested me to include him in this PM, have been able to confirmed this bug as well.
I think I found the problem, apparently the developers of the plugin removed the pre-template YouTube channel domain URL in favor of allowing the user to define their own full YouTube URL.
Thus the info text under the YouTube link should now state that is requires full URL, not just the channel name.
I finally realised this text is from my own customizations, not from the plugin that provides these links on the user card. Updated! Thanks again for reporting this.