Cannot send emails- RoundCube just shows "Sending Message"

I found a problem with Roundcube, it shows sending message and hangs up there and actually it doesn’t send message anymore. I found the following solution here.

Problem with Roundcube, installed when updated to newest CURRENT release. – Page 3 – cPanel Forums:

vi /usr/local/cpanel/base/3rdparty/roundcube/config/main.inc.php

and changed

$rcmail_config['smtp_user'] = ‘%u’;
to
$rcmail_config['smtp_user'] = ”;