In order to use the Active Directory Username Mapping Attribute option with LDAP security you must also edit the LDAP server settings to change the attribute for the user class that maps to the OnBase user so that the LDAP attribute corresponds to the Active Directory attribute being used.
When using LDAP, OnBase determines the currently logged-in Windows user and extracts the specified Active Directory Username Mapping Attribute value (sAMAccountName by default), then uses that value to query the LDAP server for a matching user.