Just copy the URL from your address bar and paste it in the message. No embed code, nothing else, JUST the url. The forum will automatically read it and do what it needs to.
remove the *'s, and replace the random number and letters, with the random numbers and letter found at the back of your video's URL. This always works for me, nothing else seems to though
Generally all you do is grab the URL for the page you're on, not the one that is generated from the "share" links or embed links etc. - copy URL from browser, paste - that's all.
I use the Insert Link button (the globe with a chain link - is that what you mean when you say "I clicked embed"?) and it works every single time.
Although, as noted, I make sure to always use the pure link (ending in 'watch?v=xxxxxxxxxx' and remove anything after and including the &; all that 'feature' and 'embed' stuff in the url probably screws it up).
I think you have to select the "long URL" option on YouTube when sharing the link, to get it to work correctly here. (You get a "youtube.com" instead of a "youtu.be" link. I can only assume that UltimateMetal hates Belgium. )
I just noticed one other thing that might be causing the problem for you. Using the Insert Link button, I just tried to paste in a regular YouTube link from the window I was in, but it wouldn't show the video window in the posting, just the url text. Then I realized it was because I was on a secure version of YouTube, requiring me to remove the 's' from the 'http' in the url.