I tried several times too and digged further into it with ldapsearch. I think the plugin does not work since the fields creacted in ldap.conf are not complete nor correct in particular when trying to bind to a TLS enabled ldap service. I can get some information when I use as root in shell ldapsearch but only if I bind as anonymous. However it does not solve my issues in my configuration since I need TLS to connect to the ldap server on another VM for autentication.
There is atleast one thing missing in the plug-in front-end: the ability to select a certificate (similar like in the ftp plugin). This would allow to link the ldap pulgin to the imported certificate which is used by the LDAP server.
Hope this feature request / bug will get some attention.
A full OpenLdap Server plugin would be a great add-on for OMV (may be something on the list for OMV5)