OK. I have set up a new MySQL Server in AWS Ubuntu EC2 and migrate databases from the RDS. After importing the tables, stored procedures and functions into the new server, I encountered the attached error.
This happens whenever I want to create or drop users, or if I modify the stored procedure and apply changes. For stored procedures or functions, if I continue to apply changes second time, it would work.
I tried to delete those users in my Workbench but the pop-up error prevents me from doing that.
I think these users were created when I import those stored procedures with those definers.