But your snipped worked. So the issue is how Mastodon builds that embed-code (or how Discourse treats it).
What ever, now I know how I must edit it. Out-of-the-box solution would be perfect, but we aren’t living in the perfect world, so… An for me that procedure is enough, because in my forum I am absolutely the only one who will embed from Mastodon, at the moment anyway.
Thanks, again.