2944571 - My Inbox substitution - user does not exists

Substitution creation (planned and unplanned) is failing in My Inbox Fiori application with the following error messages:

• Error - Cannot create substitution rule for all systems because the substitute user is missing in some of the backends. To re-create the rule, ensure the substitute user exists in all system and resend the rule.
  
  AND
  
  
• User ***** does not exists (after clicking Show Details)
  
  AND
  
  
• No substitution rule is created in the connected backend systems, even where the user exist.

• Task Gateway
• SAP NetWeaver
• SAP Web Application Server for SAP S/4HANA
• ABAP PLATFORM - Application Server ABAP
• SAP Gateway
• My Inbox Fiori Application
• Multi Origin Composition (MOC)

SAP_GWFND, IW_PGW, /IWPGW/TGW, /IWWRK/WORKFLOW, IW_BEP, TASKPROCESSING, error tolerance, SPRO, SAP NetWeaver,  Gateway,  OData Channel Next, Composition, Set Error Tolerance for OData Services for MOC, custom service document identifier, ZTASKPROCESSING_0002, TGW_STD , KBA , OPU-BSC-TGW , Task Gateway , CA-INB-FIO , Fiori UI for My Inbox Application , OPU-GW-COR , Framework , OPU-BSE-WFM , Gateway Business Workflow Enablement , Problem