If a post is started by quoting another post via the double quotes icon, then "Save Draft" will fail unless new text is entered prior to the quoted text. The error page returned says "The requested post does not exist". This appears to be caused by the redirect URL being written incorrectly, e.g.,
This does not occur if a new empty post is started and the user then clicks on the double quotes of an existing post. In that case, the post is inserted correctly and "Save Draft" works properly.