The problem is that your Host does not allow a post back to the Return link the way it is set up right now. Instead of sending the buyer back to this link:
http://www.domain.com

You should try either the Full Path:
http://www.domain.com/index.html

Or, if the index.html page causes the same problem, then maybe change the page to a specific page, and not an index page.
http://www.domain.com/page.htm