Configuring LDAP with OrientDB 3.1.9

I am trying to configure OrientDB 3.1.9 to work with LDAP, following the example in the documentation, and using this test server. My configuration is as follows:

...
  "ldapImporter": {
    "class": "com.orientechnologies.security.ldap.OLDAPImporter",
    "enabled": true,
    "debug": false,
    "period": 60,
    "databases": [
      {
        "database": "demodb",
        "ignoreLocal": true,
        "domains": [
          {
            "domain": "example.com",
            "authentication": "GSSAPI",
            "servers": [
              {
                "url": "ldap://ldap.forumsys.com:389"
              }
            ],
            "users": [
              {
                "baseDN": "CN=read-only-admin,DC=example,DC=com",
                "filter": "(objectClass=*)",
                "roles": ["admin"]
              }
            ]
          }
        ]
      }
    ]
  },
...

After a minute or so of running OrientDB (presumably this is when it first tries to do an LDAP Import), I get the following error:

SEVER {db=demodb} OLDAPImporter.importLDAP() Could not obtain an LDAP Subject for Database demodb [OLDAPImporter]

Enabling debug: true doesn’t seem to provide any further information and I have no prior experience working with LDAP, so I’m at a loss as to what the problem might be.

Any suggestions for getting it working?