This is a known problem with youtube "localising" the url to your particular locality/country.
Either you delete your youtube cookies and when going to the youtube site (www) click cancel when you see the localisation message:
Click "OK" to accept these settings or click "Cancel" to set your language preference to "English (GB)" and your country filter to "Worldwide (All)".
However, you can also do a search in the BBCodes topic
for "youtube" and there are a couple of solutions that will resolve this. You'll need to delete your current youtube BBCode and add one of the new ones in.
Des. . .