How do I display a video from a link?
video
Just paste the full link.
It will automatically generate the thumbnail.
E.g.
You’ll have to remove the [ video] tag and the parentheses enclosing the actual link.
I had this effect as well from time to time… don’t know why this is automatically added sometimes (always?).
Most probably it’s a “feature” of the Discourse software platform we’re on with the forums. Does it make a diffrence between browsers or OS maybe?
Found it: I used the hyperlink symbol in the menu instead of pasting the url directly here 
“Au temps pour moi” 