Just had a hunch and restarted ambari-server using
This is not directed in the documentation! https://docs.cloudera.com/HDPDocuments/HDP3/HDP-3.1.4/ambari-authentication-ldap-ad/content/amb_ldap...
This makes the LDAP login work succesfully!
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Nice one!! Always restart Ambari after any Ambari-server setup commands.
Very glad to see you got it!