Traffic being routed through Cloudflare is sending invalid headers to our server

What is the name of the domain?

me2you.ie

What is the error number?

401

What is the error message?

401 - Unauthorized

What is the issue you’re encountering

Server returns 401 when traffic is proxied through Cloudflare

What steps have you taken to resolve the issue?

Set the domain to DNS only

Was the site working with SSL prior to adding it to Cloudflare?

Yes

What is the current SSL/TLS setting?

Full

What is the invalid header?