2631884 - Portal Runtime error on access of backend application from portal after clearing portal cache

Portal runtime error is seen while accessing the backend application from portal after clearing the portal cache.

Below errors can be seen in default traces:

 Caused by: com.sap.security.api.UMException: The ABAP master system cannot be retrieved.

 com.sapportals.portal.appintegrator.ApplicationIntegratorException: Cannot retrieve ABAP user for this system.
 System Alias :   'XXXXXXXXXXXX', 
 User             :   'XXXXXXXXXXXXX, '. 
 Reason         :   The ABAP master system cannot be retrieved.

 The below error can also be seen in data source configuration page under "UME Configuration" in System Administration tab.

 The reference system "XXXXXXXXXX" does not exist (any more). To prevent problems with ticket-based SSO to other systems, select a valid system or "No reference system" on the User Mapping tab

• SAP NetWeaver Portal (EP) 7.3x
• SAP NetWeaver Portal (EP) 7.4
• SAP NetWeaver Portal (EP) 7.5

Enterprise Portal 7.30, EP 7.30, Enterprise Portal 7.31, EP 7.31, Enterprise Portal 7.40, EP 7.4, Enterprise Portal 7.50, EP 7.5, system alias, system object, blank, cache, personalisation UMException,The ABAP master system cannot be retrieved,ApplicationIntegratorException,Cannot retrieve ABAP user for this system, system alias blank, portal runtime error, run time error, run time issue, system alias, blank, system alias empty , KBA , EP-PIN-PSL , Portal System Landscape , EP-PIN-AI , Application Integration , EP-PIN-PRT , Portal Runtime , Problem