diff -urN ../roundcubemail-1.4.11.orig/vendor/kolab/net_ldap3/lib/Net/LDAP3.php ./vendor/kolab/net_ldap3/lib/Net/LDAP3.php
--- ../roundcubemail-1.4.11.orig/vendor/kolab/net_ldap3/lib/Net/LDAP3.php	2019-10-21 14:18:59.000000000 +0300
+++ ./vendor/kolab/net_ldap3/lib/Net/LDAP3.php	2021-09-11 16:11:09.303801000 +0300
@@ -66,6 +66,8 @@
         "v" => "read"
     );
 
+    private $hosts_used = array();
+
     /*
      * Manipulate configuration through the config_set and config_get methods.
      * Available options:
@@ -601,17 +603,29 @@
         }
 
         if (is_resource($this->conn)) {
-            $this->_debug("Connection already exists");
-            return true;
+            if ($this->_current_bind_dn) {
+                $this->_debug("Connection already exists");
+                return true;
+            } else {
+                $hosts = (!empty($host) ? (is_array($host) ? $host : array($host)) : $this->config_get('hosts', array()));
+                $hosts = array_diff($hosts, $this->hosts_used);
+                if (count($hosts) == 0) {
+                    $this->_debug("Connection already exists");
+                    return true;
+                } else {
+                    $this->_debug(sprintf("C: Try next LDAP host %s", $hosts[0]));
+                }
+            }
+        } else {
+            $hosts = !empty($host) ? $host : $this->config_get('hosts', array());
         }
-
-        $hosts = !empty($host) ? $host : $this->config_get('hosts', array());
         $port  = $this->config_get('port', 389);
 
         foreach ((array) $hosts as $host) {
             $this->_debug("C: Connect [$host:$port]");
 
             if ($lc = @ldap_connect($host, $port)) {
+                $this->hosts_used[] = $host;
                 if ($this->config_get('use_tls', false) === true) {
                     if (!ldap_start_tls($lc)) {
                         $this->_debug("S: Could not start TLS. " . ldap_error($lc));