There are two possibilities why you would receive an error message “IP address has changed” from cPanel.
1) Your Internet Service Provider (ISP) has a short DHCP lease time, causing your IP address to be renewed/released and reassigned while you are logged into cPanel/Webmail.
2) You are attempting to login to cPanel using the same account from two different devices concurrently.
1) cPanel access - https://server_name:2083/ - where you need to replace server_name with the name of the server on which your account has been activated
2) Webmail - https://server_name:2096/ - where you need to replace server_name with the name of the server on which your account has been activated
( Use the server IP or Request EACdirectory to issue the server Url )
Other things to check which might be causing this error:
1. The date on your computer is set incorrectly or in the future causing the session to expire
2. Having a software/plugin/setting that blocks third-party cookies aggressively (things like anti-virus software)
3. Your browser is not set to accept cookies (browser settings)
4. You have some sort of proxy service/ firewall configured (service like sucuri/ Cloudflare services might need to be bypassed.)
5. Your IP is dynamically changing (typically office network set up/ host IP leases to modem/router)
6. You are using Cloudflare - in this case, you need to exclude the address for your webmail/Cpanel from CF so that CF is bypassed. If you have VPS/Cloud server you can disable the IP Cookie validation