Kindly re-check if Cloudflare is allowed to connect to your origin host to as follows in the below article:
Nevertheless, Cloudflare IP addresses list can be found here:
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 … )?
Before moving to Cloudflare, was your Website working over HTTPS connection?
Can you double-check if your web hosting provider issued an SSL certificate which is still a valid one for your domain(s)?
Before doing anything at Cloudflare settings, you could determine if you have a valid SSL certificate installed at the origin host/server by your web hosting provider or your own VPS/dedicated server following the steps from below:
Use the “Pause Cloudflare on Site” option from the Overview tab for your domain at dash.cloudflare.com .
The link is in the lower right corner of that page.
Give it five minutes to take effect.
Check with your hosting provider / cPanel AutoSSL / ACME.sh / Certbot / Let’s Encrypt or some other and renew it accordingly.
Make sure site is working as expected without any errors via HTTPS.
Only then should you un-pause Cloudflare and double-check your SSL/TLS setting to make sure it’s Full (Strict).
Here is a way to re-check if you correctly setup the SSL for your domain with Cloudflare:
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 Origin CA Certificate: