Turn off HSTS, Always Use HTTPS and SSL/TLS Recommender then access your site in another device (incognito tab is recommended because otherwise browser will save ssl configuration and affect debugging)
You should use Flex mode if you don’t want to install ssl on your server. I do not recommend this.
I’m pretty sure the error is caused by “Always Use HTTPS” or “SSL/TLS Recommender” if it’s Cloudflare’s fault.
If not, it’s probably your server. Try checking any configuration on your server that automatically redirects HTTP traffic to HTTPS.
Or you can try checking your “Page Rules” if you accidentally enabled any SSL related options
You can change the URL to HTTPS now (since you using flex ssl now right?)
And whitelist your server ip and any server of whatever plugin you’re using (try bypassing the entire IP using the following rule to see if it works, delete it as soon as debugging is done)
If it works fine, you will know what the problem is.