PHP警告：ftp_put（）：无法建立数据连接：不允许操作 - 一段时间后

Question

we build a huge file transfer script that fetches ftp data from an external server and puts it to our main server. So we setup a passive ftp-connection and start the php-script via shell/cronjob.

Currently we are trying to fetch 5000 files and it breaks after 2000 files and about 3 minutes.

This is the output:

[root vhosts]# /usr/local/psa/admin/bin/php  /var/www/vhosts/domain.com/httpdocs/fetchFTPdata.php user password action
PHP Warning:  ftp_get(): Unable to build data connection: Operation not permitted in /var/www/vhosts/domain.com/httpdocs/fetchFTPdata.php on line 771
PHP Warning:  ftp_put(): Unable to build data connection: Operation not permitted in /var/www/vhosts/domain.com/httpdocs/fetchFTPdata.php on line 551

And here is how we setup the connection:

$ftp_conn = ftp_ssl_connect($url, 21, 3600);
        if (!$ftp_conn)
            return false;
        if (@ftp_login($ftp_conn, $usname, $pwd)) {
            $this->ftp_conn = $ftp_conn;
            if (!ftp_pasv($ftp_conn, $true)) {
                $this->writeLog('Passive FTP-Mode not available.');
                return false;
            }
            return true;
        }

This is how we put the files:

if (!ftp_put($this->ftp_conn, $remote_file, $local_file, FTP_BINARY))
                return ('error');

Any ideas on why this stops working after a specific amount of time?