As of WorkflowGen version 7.15.5, the optional BACKURL_SUBMIT and BACKURL_CANCEL HTTP parameters used in custom WorkflowGen URLs (for example, when launching a new request and starting the first action within another website) now ONLY support absolute URLs (starting with http:// or https://), relative URLs (starting with / ), or URLs defined in the PortalRedirectAllowedHttpUrls configuration parameter (e.g. value='blank.htm, myPage.htm').
See the Integration Using WorkflowGen URLs section in the WorkflowGen Integration Guide for instructions on how to set up and use custom WorkflowGen URLs.