Are you having an A type or a CNAME type DNS record to your Plesk?
May I ask if the Plesk is running on a sub-domain?
Are there any other app running on the same port or behind some load-balancer?
Regarding an SSL certificate, have you had one installed before moving to Cloudflare? If so, does it cover both main domain and any other possible sub-domains running?
May I ask what SSL option have you got selected under SSL/TLS tab at Cloudflare dashboard? (Flexible, Full …)
Furthermore, may I ask if Cloudflare IP address(es) are allowed to connect to your origin host/server? Therefore, I am also thinking about CF-Connecting-IP and set_real_ip_from.