What is the name of the domain?
What is the issue you’re encountering
It was possible to access the website normally before, but now it suddenly shows an error. The certificate has been verified as correct, and the cause is unknown.
Was the origin SSL certificate renewed and is it valid?
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 … )? 
Does it work Cloudflare is Paused temporary?
- 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, then make sure site is working as expected with HTTPS without any error
After pausing Cloudflare, the website becomes accessible.
Full Strict