Hi – I am Nicole – one of the developers at DukaPress.
I did have a look at your site and to be honest once we established that it is not a theme or plugin conflict, we tried hard to find what else could be causing the problem but did not find anything conclusive.
Can you try the following:
-Try upload a fresh installation of DukaPres (you probably have tried this but I wanted to mention it)
-Try and create a test site on a different server/web host. In the past we have found that sometimes things just do not work on certain hosts.
Please accept our apologies once again.