Cannot add new website suddenly (The server already host few websites.)

It was no problem when I added new sites before.
However, I cannot do that yesterday.
No any error message show in ispconfig interface, but I checked that here had no any new folder(e.g. clients/client[id]/web[id]) created by ispconfig.
I cheked MySQL, the data of the websites account is normal.
I search by web but cannot found the solution with similar case.
Please help for this,
Thank you very much

 

Thanks for your help.
when I run the command: /usr/local/ispconfig/server/server.sh , It show that:
"PHP Parse error: syntax error, unexpected '[' in /usr/local/ispconfig/server/plugins-available/mail_plugin.inc.php on line 407"
Is it the reason of I can't add a new sites?
But this mail_plugin.inc.php I had not modified by day one, how come it show error now?
BTW the following is the code:

if($mount_backup){
$sql = "SELECT * FROM mail_domain WHERE domain = ?";
*****line 407 **** $domain_rec = $app->db->queryOneRecord($sql, explode("@",$data['old']['email'])[1]); *****line 407 ****
if (is_array($domain_rec)) {
$mail_backup_dir = $backup_dir.'/mail'.$domain_rec['domain_id'];
$mail_backup_files = 'mail'.$data['old']['mailuser_id'];
exec(escapeshellcmd('rm -f '.$mail_backup_dir.'/'.$mail_backup_files).'*');
//* cleanup database
$sql = "DELETE FROM mail_backup WHERE server_id = ? AND parent_domain_id = ? AND mailuser_id = ?";
$app->db->query($sql, $conf['server_id'], $domain_rec['domain_id'], $data['old']['mailuser_id']);
if($app->db->dbHost != $app->dbmaster->dbHost) $app->dbmaster->query($sql, $conf['server_id'], $domain_rec['domain_id'], $data['old']['mailuser_id']);

$app->log('Deleted the mail backups for: '.$data['old']['email'], LOGLEVEL_DEBUG);
}
}

It seems no any error on it.
Do you have any suggest?

Thank you so much!

 

Thank you very much!
I solved the problems now