Have you quiried the system user table With Id : 8d53f9a9-1343-e211-819b-001dd8b71c08 .
what it returns?
Also have you checked, if the user which have added newly does exist in your DB already?
I mean , if you have added Domain1\Username , that user does not already exist in crm as Domain2\username. This is just an example, check if this user is associated to CRM already in some way like this.