Spring LDAP search for subgroups results in CommunicationException [root cause: UnknownHostException] - ldap

Question

We have a plain java command line software which performs a recursive LDAP search with Spring LDAP, starting from a specified group and searching all the users from the specified groups and subgroups.
The Ldap search is configured as follows:
LdapContextSource ctxSrc = buildLdapContextSource();
PoolingContextSource poolingContextSource = buildPoolingContextSource(ctxSrc);
LdapTemplate ldapTemplate = new LdapTemplate(poolingContextSource);
private LdapContextSource buildLdapContextSource() {
LdapContextSource ctxSrc = new LdapContextSource();
ctxSrc.setUrl(config.getProperty(Config.LDAP_URL));
ctxSrc.setUserDn(config.getProperty(Config.LDAP_LOGIN_USERNAME));
ctxSrc.setPassword(config.getProperty(Config.LDAP_LOGIN_PASSWORD));
ctxSrc.setReferral("follow");
Map<String, Object> baseEnvironmentProperties = new HashMap<>();
baseEnvironmentProperties.put("java.naming.ldap.attributes.binary", "objectGUID");
ctxSrc.setBaseEnvironmentProperties(baseEnvironmentProperties);
ctxSrc.afterPropertiesSet();
return ctxSrc;
}
private PoolingContextSource buildPoolingContextSource(LdapContextSource ctxSrc) {
PoolingContextSource poolingContextSource = new MutablePoolingContextSource();
poolingContextSource.setContextSource(ctxSrc);
poolingContextSource.setDirContextValidator(new DefaultDirContextValidator());
poolingContextSource.setTestOnBorrow(true);
poolingContextSource.setTestWhileIdle(true);
return poolingContextSource;
}
The software performs an empty search (in case of refused connection or connection timeout), and then starts recursive to two different groups. The software searchs for possible subgroups with lines:
AndFilter filter = new AndFilter().and(new EqualsFilter("objectclass", "group")).and(new EqualsFilter("memberof", groupName));
List<String> subgroups = ldapTemplate.search(base, filter.encode(), SearchControls.SUBTREE_SCOPE, new GroupNameMapper());
GroupNameMapper maps group's distinguished name to String. Thus far the tool has worked with handful of customers just fine, but now we encountered a new problem.
The tool makes two different searches. Admins and normal users. The customer has provided the group DNs like:
CN=SyncAdminsGroup,OU=Services,DC=example,DC=org
CN=SyncNormalGroup,OU=Services,DC=example,DC=org
Base and Organisational Unit is same for both groups. The first search for admin group works fine. No recursive search were made, so the admins did not contain any subgroups.
The second results in an error:
ERROR - Encountered error with LDAP:
org.springframework.ldap.CommunicationException: example.org :389; nested exception is javax.naming.CommunicationException: example.org :389 [Root exception is java.net.UnknownHostException: example.org ]
at org.springframework.ldap.support.LdapUtils.convertLdapException(LdapUtils.java:108)
at org.springframework.ldap.core.LdapTemplate.search(LdapTemplate.java:397)
at org.springframework.ldap.core.LdapTemplate.search(LdapTemplate.java:328)
at org.springframework.ldap.core.LdapTemplate.search(LdapTemplate.java:604)
at org.springframework.ldap.core.LdapTemplate.search(LdapTemplate.java:594)
at org.springframework.ldap.core.LdapTemplate.search(LdapTemplate.java:482)
at org.springframework.ldap.core.LdapTemplate.search(LdapTemplate.java:498)
at LDAP.LdapImporter.searchLdapGroup(LdapImporter.java:128)
...(omitted lines from the software)
Caused by: javax.naming.CommunicationException: example.org :389 [Root exception is java.net.UnknownHostException: example.org ]
at com.sun.jndi.ldap.LdapReferralContext.<init>(Unknown Source)
at com.sun.jndi.ldap.LdapReferralException.getReferralContext(Unknown Source)
at com.sun.jndi.ldap.LdapCtx.searchAux(Unknown Source)
at com.sun.jndi.ldap.LdapCtx.c_search(Unknown Source)
at com.sun.jndi.toolkit.ctx.ComponentDirContext.p_search(Unknown Source)
at com.sun.jndi.toolkit.ctx.PartialCompositeDirContext.search(Unknown Source)
at com.sun.jndi.toolkit.ctx.PartialCompositeDirContext.search(Unknown Source)
at javax.naming.directory.InitialDirContext.search(Unknown Source)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.springframework.ldap.pool.factory.DirContextPoolableObjectFactory$FailureAwareContextProxy.invoke(DirContextPoolableObjectFactory.java:271)
at com.sun.proxy.$Proxy17.search(Unknown Source)
at org.springframework.ldap.pool.DelegatingDirContext.search(DelegatingDirContext.java:347)
at org.springframework.ldap.core.LdapTemplate$4.executeSearch(LdapTemplate.java:322)
at org.springframework.ldap.core.LdapTemplate.search(LdapTemplate.java:363)
... 11 more
Caused by: java.net.UnknownHostException: example.org
In the exception, the "Unknown Host" is same as the search base, eg. CN=example,CN=org --> org.springframework.ldap.CommunicationException: example.org:389;
What could be the cause for such error? This error seems to indicate some kind of DNS problem, but since the first search with the same base works, it leaves me baffled.
Searching for the error, all others seem to encounter either "Connection refused" errors or PartialResultExceptions, which is not the case here.