If you installed the cert using the ispconfig control panel for the website knotpc.com and that site is working properly ssl in all. And you want to use the same cert for your ispconfig control panel at knotpc.com:81 you would need to copy your crt and key to your ispconfig apache ssl folders. be sure to rename then server.crt and server.key so at your next ispconfig update you wont have to redo anything.
cp /var/www/web#/ssl/www.yoursite.com.key /root/ispconfig/httpd/conf/ssl.key/server.key
cp /var/www/web#/ssl/www.yoursite.com.crt /root/ispconfig/httpd/conf/ssl.crt/server.crt
then restart ispconfig