You are using POWL only then with no WF?
You need to check your config then
MSS NAV
what is the system alias maintained?
System Alias SAP_ERP_HumanResources
and check your config in Systems admin in Portal?
You can do a HTTPtrace and see what parameters are passed and it ll be easier to troubleshoot then
Above error is typically due to incorrect parameters or wrong system alias, as it works from backend
POWL Inbox configurations for Manager Self-Service EhP5 and above