User not registered in LDAP server

I've just setup ldap and all seems to be working.

This is my ldap configurations:

   "LdapSettings": {
        "Enable": true,
        "LdapServer": "spkmain01.spkanim.com",
        "LdapPort": 389,
        "ConnectionSecurity": "",
        "BaseDN": "dc=spkanim,dc=com",
        "BindUsername": "spkanim\\testusersg",
        "BindPassword": "password",
        "UserFilter": "(objectCategory=Person))",
        "FirstNameAttribute": "givenName",
        "LastNameAttribute": "sn",
        "EmailAttribute": "mail",
        "UsernameAttribute": "sAMAccountName",
        "NicknameAttribute": "nickname",
        "IdAttribute": "objectGUID",
        "SyncIntervalMinutes": 2,
        "SkipCertificateVerification": true,
        "QueryTimeout": 60,
        "MaxPageSize": 500,
        "LoginFieldName": ""
    },

It seems to synchronized successfully:

[2016/07/30 12:38:24 SGT] [INFO] LDAP Synchronization completed

However, when I tried to sign on with a AD user, I've got the these errors:

[2016/07/30 12:38:34 SGT] [EROR] /api/v3/users/login:findUser code=400 rid=nhnj6ji7uj8cddaeh7hwmodmfw uid= ip=10.0.17.5 User not registered on LDAP server [details: username=spkanim\savio]
[2016/07/30 12:38:38 SGT] [EROR] /api/v3/users/login:findUser code=400 rid=57w7xswijpd89crjenkzap6yqy uid= ip=10.0.17.5 User not registered on LDAP server [details: username=savio]

I've tried a few other users and the results are the same. Is there anyway to see how many users are synced to Mattermost from ldap? I've also gone through the troubleshooting section in the docs but none of that helps. Did I miss anything?