In AEM 6.1, service users must be system users, which effectively means that their node in the JCR is of type rep:SystemUser. These users cannot be used to log in normally, only by background processes. The admin user is not a system user, so you cannot use the admin user in a service user mapping like this. You have to create a new system user and assign them the appropriate permissions.
If you would like to read more of the background on this change, take a look at https://issues.apache.org/jira/browse/SLING-3854.