I contacted my host and they told me that everything is fine on their side, including my SSL certificate is valid and with AutoSSL in Cpanel.
I see you have a new SSL certificate issued by cPanel on the 30th of September. So perhaps the previous certificate had expired after all?
You currently have a proper CA-issued certificate at the origin. Change your Cloudflare SSL/TLS setting to FULL (Strict). You should not have the problem again when you turn Proxy () back on.