What is causing wp-comments-post.php to redirect to the browser’s IP address?

I’ve got a copy of WordPress running that has recently been moved off shared hosting and onto a VPS that I control. While a testing copy of the same site runs correctly on another vhost on the same instance, on the same IP address, after the time when I moved the public site across, the comments form began to redirect visitors back to their own IP address with a 302 Found in place of the successful 302 Moved Temporarily.

Running WP 3.4.1, I’ve tried:

• reinstalling WP from the dashboard, and checked all plugins are up to date
• deactivating all plugins and setting the theme to twenty-eleven
• diffing against the working test installation to check they are the same
• testing from more than one IP, computer and browser

all with no effect to the problem.

What is causing WordPress to redirect from the form submission back to the browser’s ip address?

Here’s a sample request to leave a comment at http://def-proc.co.uk/b/jwybj:

Request URL:./blog/wp-comments-post.php
Request Method:POST
Status Code:302 Found

Form Data

author:My Name
email:email@my.mail.box
url:(empty)
comment:Comment contents.
akismet_comment_nonce:(akismet nonce)
submit:Post Comment
comment_post_ID:110
comment_post_ID:110
comment_parent:0
bb2_screener_:(bb2 nonce) (my ip address)

Response Headers

Connection:Keep-Alive
Content-Encoding:gzip
Content-Length:243
Content-Type:text/html; charset=iso-8859-1
Date:Wed, 01 Aug 2012 17:34:41 GMT
Keep-Alive:timeout=15, max=100
Location:http://(my ip address)/ 
Server:Apache
Vary:Accept-Encoding