Furthermore, was the Website working fine over HTTPS before moving to Cloudflare?
May I ask do you get this error/message in Mozilla FireFox or Google Chrome Web browser?
Was the domain nameservers recently changed? Usually the DNS changes fluctuate for the first 24 hours.
I suggest try clearing your Web browser cache, or DNS cache by restarting your router, or try accessing your Website using another Web browser or even a private Window, or via a VPN connection, if possible, to see if you get the same error.
I believe it has to be related to the HTTPS connection with your Website which, hopefully, is using Cloudflare and is being cloud (proxied).
Therefore, you would have to have a valid SSL certificate which covers both your non-www and www domain (and some other potentially), kindly check the SSL/TLS tab at Cloudflare dashboard for which option have you got selected there as it should be Full (Strict) SSL.
Kindly take some time and re-check the SSL settings from the below article how to propperly setup SSL with Cloudflare for your domain:
Available SSL options at Cloudflare are here:
If any other issues appear, follow the needed steps for troubleshooting from article below:
If you want to use Cloudflare SSL, kindly make sure you follow the instructions as follows on the below article to setup an SSL certificate using Cloudflare CA Origin Certificate:
cloud (proxied).