I've just had a light bulb moment, lol. The characters I listed that are causing the videos to fail...
a A c C h H t T v V w W
...are the same characters (ignoring the case) that are between the .com/ and the ID of the video URL...
https://www.youtube.com/watch?v=YytFr9d_otQ
...okay, there is an equals sign and question mark in there as well but as I have never seen those in a video ID, I never included them in the list of characters. But, putting them in the video ID just to test it, also causes the video to fail.
So, the code that WGT is using to convert the video URL we use to post a video...
https://www.youtube.com/watch?v=YytFr9d_otQ
...into the one they use to put in an iframe element to put in the forum...
https://www.youtube.com/embed/YytFr9d_otQ
...appears to be searching for the characters 'w, a, t, c, h, ?, v, =' and removing them from the video URL, but it's also removing those characters if they occur at the front of the video ID, and if that happens, the video will fail because it won't have 11 ID characters.
Okay WGT, I've given you enough clues, it's up to you to mend it now!