The 526 error refers to the certificate on your origin. If it was working before, then likely it has expired and needs renewing. Otherwise, make sure it is not self-signed and covers the hostnames you are using.
It will because Cloudflare is no longer checking the origin certificate is valid, that is insecure.
It can’t, Cloudflare handles the edge SSL, you need to handle the origin SSL. Either renew the certificate or use a Cloudflare origin certificate to ensure your site is fully secured.