Red Cross or Blank compose window in Outlook Web Access

This I have seen happen many times and the answer is very simple, It mostly happens on clients running Windows Vista, you will find that in Vista, Microsoft changed the way it uses the Active X control and so you need to update the server with a more recent component.

This applies to Vista and Exchange Server 2003

This update is freely available from microsoft here:
http://support.microsoft.com/kb/911829

P.S I Have seen this when you are using IE7 on XP for some reason so this may still apply if that is the case with you.