ORA-20002: [WF_NO_USER] NAME=XXXXX ORIG_SYSTEM=NULL ORIG_SYSTEM_ID=NULL has been detected in FND_USER_RESP_GROUPS_API.INSERT_ASSIGNMENT

I got the above error when trying to add responsibility to a FND User.

There is no any specific reason for this,

As suggested by Corvin below, do the dummy update to the 'person' field

Login to the application and query the user
Clear the 'person' field and save, re-enter the the person field and save

Try adding responsibility.

If above steps does not resolve the issue please perform below steps

1>  Run the program "Synchronize WF LOCAL tables"
      
       Parameters:
       Orig System: ALL / Specific System (Ex: FND_USER)
        Parallel Processes : Number of parallel processes
        Logging Mode:
        Temporary Tablespace:
        Raise Errors:

> Only "Orig System" is a mandatory parameter

Alternatively you can execute below script

BEGIN

  WF_LOCAL_SYNCH.BulkSynchronization('ALL');

EXCEPTION

   WHEN OTHERS THEN

      DBMS_OUTPUT.PUT_LINE('ERROR: '||SQLERRM); 

END;

>> Check if the issue resolved, if not follow to step 2

2> Run the program "Workflow Directory Services User/Role Validation"
     > Change parameters 'Fix dangling users'                                -> Yes
                                             'Add missing user/role assignments' -> Yes
     > Leave other parameters with default values
     > If you want to run for a specific User/Role, please provide the related parameter value

> I hope this will resolve the issue.