Hi Dean
I added www to the address, to listmail files as described here
If this happens even using the preview form I would suspect a rare issue with the ListMail URL. Some servers due to configuration cannot accept form POSTs to certain forms of URLs... My suggestion is this: If the ListMail URL contains "www.", try removing it. If the URL does not contain "www.", try adding it.
That worked OK on the admin signup test, but on the website it was still the same as before
Regards
Adele