Hi Leonardo,
As per our conversation I believe the WebDynpro application responsible for the approval of Change Requests in MDG it is not considering the Work Item ID as being passed by TASKPROCESSING service - it only works if the app is being called with the Change Request Number (CREQUEST).
Back in the old Portal days, UWL was only able to "see" the Work Item attributes from the container. And when you had to work with additional attributes from a BOR object like BUS2250 (MDG) you had to enter that information on SWFVISU so that UWL would be able to look inside the BOR object and retrieve the Change Request Number to be sent to the application for approval.
POWL work in a different way to setup the Webdynpro Dynamic Parameters. And that's the reason why POWL is able to open the Work Item. Therefore, I believe that SWFVISU will have no impact on POWL. I also suspect that if you had configured UWL with this backend system it would show you the same error.
The following link shows you how POWL works to determine the application to be opened.
How to enable the existing Webdynpro based workflow inboxes to execute ALL tasks without config!
However, Fiori works with TASKPROCESING and this service will look into it the configuration options entered under SWFVISU. Thus, this is the only way for you while working with MyInbox.
With that in mind you need to make sure that the WebDynpro App is capable of candling the Work Item ID via the parameter ${item.externalid} and open the corresponding CREQUEST. The parameter for app USMD_CREQUEST_PROCESS should be CREQUEST_WORKITEM=${item.externalid}. Since you have already tested this directly on the browser and the app doesn't show you the Change Request, I suspect this is a programming error.
I would start by checking if the following notes applies to your system. All of them deal in some way with this application:
http://service.sap.com/sap/support/notes/1761433
http://service.sap.com/sap/support/notes/2012229
http://service.sap.com/sap/support/notes/1938716
http://service.sap.com/sap/support/notes/1996913
http://service.sap.com/sap/support/notes/2122551
If after applying relevant notes to your system the problem persists, please open an Incident/Ticket on SAP's Support System for further investigation by the team responsible for MDG .
Regards,
Ivan