Hi,
ldap.synchronization.java.naming.security.credentials parameters is only used by Alfresco to get authenticated on LDAP and be able to make LDAP request. Those credentials are not used when a user logs in Alfresco.
Could you give us the value of your authentication.chain parameter. I think it should be something like :
authentication.chain=ldap1:ldap,alfrescoNtlm1:alfrescoNtlm
With this configuration, Alfresco first checks login/credentials in LDAP and, if it doesn't match any LDAP account, it checks in local user accounts.