dasonix,

Have you tried completely removing SEO URLs? I would advise getting things working without the SEO mod installed before going any further.

I'm not familiar with the SEO mod so I cannot comment on how it does or should work. I believe that there are other contributions which do not work with this mod - e.g. Admin Profiles.

MC_callback is equivalent to the following URL

Code:
www.mydomain.com/index.php?main_page=wpcallback&zenid=dirtylongalphanumeric
Two things to note:

1. Although the ZenCart WorldPay module is based on the OSC module it differs in the way it responds to the callback response from WorldPay. The original OSC module returned the customer to a 'Checkout Success' page requiring the customer either to click on a link or not close the browser window for at least 10 seconds to allow an auto re-direct to kick in and for the order creation process to be completed. If the customer on seeing the 'Success' message simply closed the browser then the order would not be created and, as far as the merchant is concerned, lost even though WorldPay had debited the customer's card.

The ZenCart WorldPay module does the required processing to complete the order before displaying the 'Success' message which eliminates this problem. Because there is some code involved the callback URL cannot be to a .html file.

2. Although ZenCart itself is derived from OSC the two projects diverged some years ago. I cannot speak for OSC but ZenCart has developed considerably since then and I would be surprised if there are not by now some fairly fundamental differences between the two sets of code which may well render any comparison on how a module might work in either pretty meaningless.

I know that some have had problems with the WorldPay module where their server is set up with any sort of re-direct arrangement hence my comment about there perhaps being a server issue. I understand WorldPay treat re-directs with suspicion and refuses to complete the callback. This may be also an issue with the SEO URLs mod.

If, after you have removed the SEO mod, you are still getting errors please post details here and I will try to assist you as best I can.

Regards,

Alan