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:
cPanel won't load or keeps logging me out after enabling Cloudflare