Webservers and browsers can use the Diffie-Hellman key exchange method to establish secure connections. In some occassions, you may have insufficiently secure parameters for the Diffie-Hellman key exchange. While Plesk has a strong DH parameter by default, in some occassions you may want to increase the DH key exchange key size.

To increase the DH key size parameter, you will need root terminal access and you need to restart the webserver. This can be done using the following commands:

plesk sbin sslmng --strong-dh --dhparams-size=4096
systemctl restart httpd # Run this when you have Apache
systemctl restart nginx # Run this when you have Nginx
When you are unsure about which webserver you are using, it is safe to run both commands. One of the commands may return an error explaining that the service does not exist. You can ignore this safely. If a different error is returned, you will need to debug the issue. It is likely this issue already existed before increasing the Diffie-Hellman key exchange size.