Today I came accross a problem that can occur if you use the multi origin feature of SAP Gateway and if you have upgraded to SAP NetWeaver 740 SP13.
Strictly speaking it would also occur if you have upgraded solely the software component SAP_GWFND to the Support Package Level SAPK-74013INSAPGWFND.
A request such as /sap/opu/odata/sap/gbapp_poapproval;mo/WorkflowTaskCollection in this case does not return any data.
On the other hand $count would still return a result when running the following request in the SAP Gateway Client /sap/opu/odata/sap/gbapp_poapproval;mo/WorkflowTaskCollection/$count.
As a result of this strange behavior SAP Fiori applications such as the MyInbox would not work correctly.
Fortunately there is a solution 
.
Simply apply SAP Note 2250491 - OData request does not return entity set data in case of multi-origin composition to your system.
Best regards,
Andre