If I remember correctly, it has to do with the callback URL setting in the WorldPay administration panel; although the URL can be overridden, the "http" or "https" part is based on what you have set in your configuration area. I don't have an active WorldPay account, so I can't give you specific details - the following comes out of the GForum manual:
WorldPay - In your Installation Configuration menu, you should set the “Callback URL” field to contain your gforum.cgi URL.
- The “Callback enabled” checkbox must be checked.
- The “FuturePay callback Enabled” checkbox must be checked if you are going to be using your WorldPay account for recurring payments.
- If the “Use callback response” checkbox is checked, the user will be shown the worldpay_receipt.html template on WorldPay’s site upon completion of a payment. If the checkbox is not checked, WorlPay will direct the user elsewhere upon completion of a payment (this location can be defined in the Payment Page Editor section).
In case this doesn't help you, I've also sent a PM to you of someone who might be able to assist you with the WorldPay configuration.
Jason Rhinelander
Gossamer Threads jason@gossamer-threads.com