After many, many attempts, I finally succeeded in adding a reply to this thread. Maybe you can see my many failed attempts in your administrator logs.
Anyway, today I found out why my post kept disappearing, even though it did appear in the “recent activity” column on the right-hand side of the forum’s homepage.
My post seems to disappear as soon as I create a link (like the one in the first sentence in this post) to something on Youtube. It doesn’t matter whether I start the link with http or https – it always makes my post disappear.
I’ve solved the problem for now by just copy-pasting the literal URL into the post, instead of creating a nice hyperlink. But maybe this is one of the remaining bugs that you could look into.