It shouldn’t matter if you’re replying to a post or creating a new topic, but admittedly I haven’t tried the method with a new topic.
Have you tried hiding the preview window? That will eliminate the need to do things quickly. Write the text-only part of your post, then click “<<” to hide the preview, then paste in the YouTube link. Then click the “Create Topic” button to submit the post.
If you still get the error after that, it probably means that someone, somewhere already posted the link. In that case, try the trick where you append a “&1”, etc. to the link. Maybe go with something large like “&123456” if you want to be sure you aren’t colliding with somebody. The number doesn’t matter; it just has to be distinct from any other link that’s been posted.