the only way to know for sure if it is a problem with the style is to switch over to prosilver and see if it works there.
most likely it is a problem with the youtube bbcode itself.
there are some older bbcodes for youtube that are no longer viable, there was some significant changes to the way bbcode work in 3.2 etc.
have to figure out which is causing the problem.
try the prosilver first. if that doesn't work then show us the exact bbcode and html replacement that you are using for youtube.
I am available for custom work on a donation basis. Please send me a PM with your needs.
Premium phpBB 3.2 Styles
OK, so what's the speed of dark?