For user domain that are using CloudFlare nameserver, you might be facing the below issue:
Login into CloudFlare DNS panel and update cpanel.YOURDOMAINNAME.com and webmail.YOURDOMAINNAME.com to hosting server IP. Upon update, may browse both cpanel/webmail.YOURDOMAINNAME.com to login into cPanel/Webmail end where the layout will resume normal.
Official CloudFlare KB article:
NOTE: If user are unsure on the hosting server IP, kindly submit case to our support department (Submit Case) so that we are able to further check and advise on.