Paypal redirect error - redirect ok for checkout but not for pay later invoices

Has anyone had success in people paying paypal after an order?

We have paypal set up perfectly for checkout. Customer pay then they come back to our website and see they have paid.

But if the customer gets an order and then logs in online to pay it (e.g http://bit.ly/Uc85R5), Paypal will take them to their payment page, take the payment but when it takes them back to our site an error message comes up:

ERROR: An unknown error occurred. This is most likely due to the merchant profile not being set up correctly. Please contact your system administrator for help.

The payment works though, it's just the redirect page confirming their payment. I think the reason for the error is something to do with the code.

https://ourwebsite.worldsecuresystems.com/PaymentProcess.aspx?paypal=success&tx=2D355607H4466902T&st=Completed&amt=0.02&cc=AUD&cm=1 3031601|ff02efc9-0d08-47f6-8465-da202fadf4f5|eedcf84b-ce7e-4ea6-a8d1-8 ed23da22f35|10%2f17%2f2012%2003%3a35%3a49&item_number=1817147

All paypal settings are fine and as per business catalyst knowledge base.

- The only thing I can think to do is create an item called "Payment" and send the customers the Payment link to pay for their order - ie adding the payment item to a new cart and checking out like that. Any advice?

Best,

DD