Hi Huw,
It sounds like you haven't fully installed the software.
Reinstall the software FROM SCRATCH.. paying particular attention to the modifications required for the file includes/functions/html_output.php
You've either missed modifying the above file (so you've probably missed others too) or your site uses hard coding for its Add to Cart forms (unlikely)..
Either of these situations causes the software to redirect to the static URI for a site after a post to its old dynamic and therefore lose the contents of the POSTed form (meaning that it can't add the product to the cart).
All the best..
Conor
ceon
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