I've no idea. This works fine in all my tests on 2 local installs and our demo site. Ensure CB Activity is up to date. If you're using a custom CB Activity template then change it back to default to see if that resolves it. You can try clearing the URL cache and making a new post with the URL again by deleting the below folder.
/cache/activity_links
Without a means for me to reliably reproduce the issue and no errors on your end there isn't really a way for me to track down why this fails for you. At the very least it should still detect that the youtube URL is a video and output the video embed. It should parse fine as long as the cURL request to the supplied URL replies with a 200 response code.
Will review improving the error output for attachment parsing so that maybe it can be more clear why a URL fails to parse.
forge.joomlapolis.com/issues/7902