What do you type into the Email or LDAP Username field on login?
It should be just his username without a domain. So for mattermost\bill I would just type bill.
Maybe remove the object filter until you have basic login working.
Also, not sure about "BaseDN": "dc=domain,dc=com", might need to have a organization unit like "BaseDN": "ou=group,dc=domain,dc=com",