EMBED VIDEO

Do as following:

-1. Copy the iframe embed code from youtube
(which looks like this: <iframe width="420" height="315" src="//www.youtube-nocookie.com/embed/IWsQgmq-fNs" frameborder="0" allowfullscreen></iframe> )

-2. Paste it to this online html-editor , or  https://htmlcodeeditor.com/   (or another editor that outputs the html).

-3. Copy the output of the editor (select bottom window and ctrl-a to select all, crtl-c to copy)

EDIT: to select the bottom window (which shows the video), just move the mouse pointer into the window and click.

-4. Paste it to your forum post

(sometimes i also put a few fullstops [...] at the end into the  forum post).

- If the video does not show, reload the page. 

I use Chrome as browser, also.

...

If i understand well, you are doing only the 1st step (copy the iframe embed code from you tube). 

I did it just right now again. Eureka! Before you klick on embed, paste the code in the small long frame: source. Then you klick on the word embed, and then on OK.

Success!