Maybe something about Firewall or about if Cloudflare cannot connect to your new server.
Furthermore, may I ask before moving to Cloudflare, was your Website working over HTTPS connection?
And is your new server working over HTTPS too (having a valid SSL certificate installed at your new Web server covering your domain and sub-domain(s))?
If so, kindly I would recommend to re-check if Cloudflare is allowed to connect to your new origin host / new server to as follows in the below article:
Nevertheless, Cloudflare IP addresses list can be found here:
Regarding the HTTPS and the SSL certificate, may I ask what SSL option have you got selected under the SSL/TLS tab at Cloudflare dashboard for your domain ( Flexible, Full, Full Strict … )?
Here is a way to re-check if you correctly setup the SSL for your domain with Cloudflare:
Regarding available SSL options at Cloudflare dashboard, check here:
If any other issues appear, follow the needed steps for troubleshooting from article below:
In case you do not have an SSL certificate, you can use Cloudflare SSL, if so, kindly make sure you follow the instructions as follows on the below article to setup an SSL certificate using Cloudflare CA Origin Certificate: