"Content is not allowed in prolog" is a general XML error and it usually means that you have whitespace before the beginning of the content so instead of
Code:
<?xml version='1.0' encoding='utf-8'?>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
.... yadda yadda yadda ....
You may have
Code:
(new lines or tabs )
<?xml version='1.0' encoding='utf-8'?>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
If the error gave a particular file - check that file.
EDIT: ... alternatively ... it's not about your ePub at all, but is a problem sending the request to the Apple servers (firewall, et al inserting content into the body).