Why do I have this error JBAS015280 when adding a user to the Management Realm?

Issue

I recently upgraded my JBoss setup to version 6.2.1 and now when I try to call add-user.sh script I get the following error:

What type of user do you wish to add?
 a) Management User (mgmt-users.properties)
 b) Application User (application-users.properties)
(a): a

 * Error *
JBAS015280: Different realm names detected 'ManagementRealm', 'null' reading user property files, all realms