You mau be facing identified issue :
http://www.projeqtor.org/en/kunena-2/5-ask-questions/6539-solved-problem-ldap-search#10905
(fix is proposed in the post)

Quick fix (as a workaround) is to change line 2665 in /model/persistence/SqlElement.php from
if (getSessionUser()->id) {
to
if (1) {

I saw almost all the posts regarding LDAP issues and tried all the proposed solutions including this one but none of them worked for me.

I am not sure what else to check and posted the log here.

Thanks
Thirumal

It seems your search filter is not correct : does not retrieve a user or returns several users

You can add debugLog() lines or traceLog() lines in /model/User->autheticate)( methode to chech used filter and exact issue.
Something like this (line 939)

			$result_user = ldap_get_entries($ldapCnx, $result);
			if ($result_user['count'] == 0) {
			  traceLog("authenticate - Filter error : ldap_search returned no result for filter $filter_r)" );
			  $this->unsuccessfullLogin();
				return "login";
			}
		  if ($result_user['count'] > 1) {
		    traceLog("authenticate - Filter error : ldap_search returned more than one result for filter $filter_r)" );
		    $this->unsuccessfullLogin();
        return "login";
      }
			$first_user = $result_user[0];
			$ldap_user_dn = $first_user['dn'];
      if (strtolower($ldap_user_dn)==strtolower($paramLdap_search_user)) {
      	traceLog("authenticate - Filter error : filter retrieved admin user (LDAP user in global parameters)" );
      	$this->unsuccessfullLogin();
      	return "login";
      } 

it worked with sAMAccountName again