SE80 -> IBO_WDC_INBOX -> WebDynpro Component -> IBO_WDA_INBOX -> MANAGER_MSS_WDAC_POWL_INBOX ... which triggered the work overview page successfully with Notification, pending request, etc., are you using standard, can you ensure this step? The WD Application IBO_WDA_INBOX must be executed with appropriate application configuration like - HRMSS_AC_POWL_INBOX_2. If you open this application configuration from SE80 you will find that there is a component configuration defined for POWL component HRMSS_CC_POWL_INBOX_2. In this configuration there is application id specified as MANAGER_MSS_INBOX_3 or MANAGER_MSS_INBOX_2. This application id is the powl application id that you use in POWL configurations. this is how the inbox component comes to know about POWL application id.
Can you check the system alias? Define properties for actions in the maintenance view IBO_C_WF_APC. The IMG path to this customizing is: Cross-Application Components -> Process and Tools for Enterprise Applications -> Inbox ->Define Tasks and Actions it should be SAP_ERP_Humanresources as per the blog