Webmail / Cpanel shows error: There is a problem with this website's security certificate..This warning shows because Cpanel / Webmail is configured to work by the server's shared / self signed SSL. This is not a security problem and you can safely disregard it.
Such a warning will not show if you use the Non-SSL protected pages at:
http://cpanel.servername.com (For example: cpanel.siteground131.com)
http://webmail.servername.com