Shipping not required - checkout failed

I have some products in my store that are e-vouchers and vouchers that are picked up instore. I have set these products to not have shipping enabled in the product admin, and when you go through the checkout it shows as Shipping: Not required so all looks like its working fine.

I get to the DPS Payment Express payment side and that all goes through fine, and then click next to finish the order and it returns me to a checkout failed: General PxPay Error page at this url:

/Plugins/PaymentPxPay/Return?result=000003007102000900e2a497c25d7bdf&userid=OurAccountName

BUT the order is recorded in the admin and the emails all get sent out (order receipt and e-voucher emails)

I've got the store setup to skip the shipping method step if only one method available (we only offer one shipping method for our shipping enabled products)

If I put through an order for a product that has shipping enabled, the checkout completed page works fine so seems to be a problem with only products that have shipping disabled.

I get the same results whether logged in or checking out as a guest.

Does anyone have any ideas on how to fix this please?

Thanks!

Any ideas on this please?! Need to launch this week and just can't figure it out... it's working on our local test site but the emails don't send off there (just a server problem our side). Settings are all the same that I can see - the only difference is the emails don't send off the local site.