5006: Unable to redirect to vendor website. The provider failed to provide a redirectURL

I have integrated sagepay and everything seems to work, but after I put my cart on the sage payment server, I get this error in the browser:

Error code: 5006 Error Description: Unable to redirect to provider website. The provider was unable to provide a redirectURL.

My request provides a valid return url , but the response doesn't reach my action . I verified this by sending an email on the first line of the action (this email was not sent). I don't understand the reason. can anyone help me?

UPDATE The same error occurs with the pay-step starter kit (server itegration) configured with the same backlink. is this a configuration problem for my server?