The problem seems to be occurring for all accounts accessed through domain name:
But not when logging into cPanel through the server’s hostname at https://server.example.com/cpanel instead.
This appears to be caused by “domain.com” using CloudFlare’s IP address for its A record and conflicting with the “Cookie IP validation”.
You should do the following:
WHM > Tweak Settings Cookie IP validation > disabled
If you’d like to continue using “Cookie IP validation” on your server, users will need to access cPanel through the server’s IP or hostname directly to circumvent this issue.