cp_errordocument.shtml (Port 443)

Write blew code in your .htaccess file

php_value max_execution_time = 300
php_value post_max_size = 256M
php_value upload_max_filesize = 256M

and save it.

or,

1. Trying to log into Admin
2. Trying to get to checkout after adding to cart.
oz
[SOLVED]
The certificate was not installed correctlky. all okay now.
SECRET MYSTERY