This issue is caused when a user logs in, and then the IP address that they are accessing cPanel from changes.

This change is detected by a setting named 'Cookie IP Validation'.

WHM: Home -> Server Configuration -> Tweak Settings
Option: Cookie IP validation

https://docs.cpanel.net/whm/server-configuration/tweak-settings/#security

Scroll down for 'Cookie IP Validation' for more information.

In Tweak Settings, by default, this setting is set to 'strict'.

Changing the value to 'loose' should allow them to be connected.

 

Here is a response from another user and maybe you will understand it :

HI,

This issue typically occurs when trying to login to your cPanel through a proxy or an Internet Service Provider with a dynamic IP Address that is changing very quickly, due to the strict validation we have in place for security. Here are some items that will help to resolve the issue:

  • Do not use a Internet Service Provider Proxy Service to access your cPanel.
  • Uso https://yourdomain.com/cpanel o https://admin.yourdomain.com if using a cdn/firewall on your domain name) to login to your cPanel.
  • Ensure that your browser is set up to accept cookies for the cPanel you are trying to access.
  • Ensure that you do not have any browser extensions that are blocking cookies.
  • Ensure that you do not have any browser extensions that are acting as a proxy service.