I stumbled upon this thread, with the same error.
Aldo this is a long time ago I still want to give the solution.

1) The LDAP Server needs to direct to the Server that can query the user. (which is the sys id of the record ldap server)

2) The source is the DN of the user for instance in my case (ldap:uid=newton,dc=example,dc=com)
I've used a test ldap from this website (https://www.forumsys.com/tutorials/integration-how-to/ldap/online-ldap-test-server/) which can even used with the UID.

Here comes the part that fixed is for me:

3) In the LDAP OU Definitions with the Table sys_user must QUERY the User ID from ServiceNow you want.

So the "Query field" must be the field for the 'User ID' in ServiceNow

- sAMAccountName(Default) or UPN or mail

- For my instance I used 'mail' since our User ID's are equal to the 'email' (transform map)

Be sure you define your Transform maps correctly, like filling the DN and LDAP Server of the user.

If the sys property (glide.ldap.user.autoprovision) is set to true, it will insert user when not existing this will be filled in automatically aswel with the use of the transform map given. If not correctly the user can login 'once' and then fail, because of the wrong LDAP Server OR Source.

Hope this helps!