Why am I receiving approval through the Paya Exchange Gateway, but it is not sending any data to my Postback URL?
Please contact your System Administrator to pull a server log. Depending on the results, this may be a firewall issue on your end.
A workaround can possibly be to turn off Request Validation on your Response Handler page (add ValidateRequest="false" in the Page declaration), you should be able to receive the response strings from the frmSinglePayment page successfully.
*This is a workaround for programmers using ASP.Net 2.0 Web App.
**For ASP.NET MVC4: add the property [ValidateInput(false)] on the Action, and set <httpRuntime requestValidationMode="2.0"/> in the web.config file.
If you have questions or require additional information, please submit a ticket and we will be in touch with you shortly.