Failed to connect to MySQL server: Access denied

I went through the documented setup process for installing percona-xtrabackup. That went as expected.
I then created a new db user for the backup process including:
GRANT RELOAD, LOCK TABLES, REPLICATION CLIENT ON . TO ‘bkpuser’@‘localhost’;
FLUSH PRIVILEGES;

Whenever I run the backup process with that new user, it fails. However, if I run it as the db root user, it’s successful. Here’s the error I see when running it as the backup user account:
160617 08:14:57 innobackupex: Starting the backup operation

IMPORTANT: Please check that the backup run completes successfully.
At the end of a successful backup run innobackupex
prints “completed OK!”.

160617 08:14:57 version_check Connecting to MySQL server with DSN ‘dbi:mysql:;mysql_read_default_group=xtrabackup;port=3306;mysql_socket=/var/run/mysqld/mysqld.sock’ as ‘bkpuser’ (using password: YES).
Failed to connect to MySQL server: DBI connect(’;mysql_read_default_group=xtrabackup;port=3306;mysql_socket=/var/run/mysqld/mysqld.sock’,‘bkpuser’,…) failed: Access denied for user ‘bkpuser’@‘localhost’ (using password: YES) at - line 1314.
160617 08:14:57 Connecting to MySQL server host: localhost, user: bkpuser, password: set, port: 3306, socket: /var/run/mysqld/mysqld.sock
Failed to connect to MySQL server: Access denied for user ‘bkpuser’@‘localhost’ (using password: YES).

I can login without error to mysql using db user credentials for bkpuser, so I don’t think it’s the credentials.

I checked the logs but all I see are basic error entries: Jun 17 08:14:57 web06 mysqld: 2016-06-17 8:14:57 140128666168064 [Warning] Access denied for user ‘bkpuser’@‘localhost’ (using password: YES)
Jun 17 08:14:57 web06 mysqld: 2016-06-17 8:14:57 140128666471168 [Warning] Access denied for user ‘bkpuser​’@‘localhost’ (using password: YES)

Any ideas what would be causing the access error, and more importantly how to resolve it?

Thanks