A CNAME Cross-User Banned error is due to Cloudflare's prohibition on CNAME records pointing from one Cloudflare user into another Cloudflare user's account.
Cloudflare blocks these cross-user CNAME records to ensure the security of Cloudflare customers. Cloudflare applies security and performance filtering to incoming traffic at our globally distributed edge based on individually configured zone settings, and uses the incoming host to determine which settings to apply.
CNAME records are permitted within a domain (e.g. www.example.com CNAME bigserver.example.com), and across zones within a user account (e.g. www.example.com CNAME www.example.net).
Incoming CNAME records from other user accounts can be permitted for Professional, Business, and Enterprise customers by contacting support.