There seems to be a bug with some Youtube clips in that if their address includes certain non-alpha/numerica characters such as an underscore(_) it does not get posted correctly via insert/edit video, For instance https://www.youtube.com/watch?v=WlI_NQHZBTw just results in:

Until this is sorted out, if this happens to a clip you are trying to post the simpler option is to just post the link in the reply,

There is a way to embed the clip but it's more complicated. On the Youtube page with the clip click on Share (which is under the poster's name). Then from the menu which appears click on Embed which is shown above the row of sharing options. Copy the embed code which appears. It starts and ends with iframe.
Come back to the Add Reply box here. Click on Source code icon which is the last one. If there's any code shown in the box you then see, press return to get to a new line. Then paste the code into that box. Press OK to return to the normal Add Reply box

If you want to put some more text after the clip it's best to first click on the white space just to the bottom right corner of the clip and you should then see a blinking cursor. Then press return to get to a new line.

Although more longwinded it does work and is also the way you need to post any video clip on the web which is not a Youtube one.