At Cloudflare on Crypto section select Full and check if it works.
If not then check again the tutorial step by step....
There is an issue with your certificate at your server:
Error 526: Invalid SSL certificate
Error 526 happens when Cloudflare is unable to successfully validate the SSL certificate on the origin web server and the SSL setting in the Cloudflare Crypto app is set to Full SSL (Strict) for the website.
https://support.cloudflare.com/hc/en-us/articles/200721975-Error-526-Invalid-SSL-certificate
