I wish I knew enough to understand what I read on your link LOL.
I deleted the Let's Encrypt SSL and reinstalled it and now CA is working again.
FWIW, Siteground replied to my ticket with them about this issue:
The error indicates that your website is attempting to fetch a resource via an unsupported protocol. Usually, the error is present when an application attempts to use TLS 1.0 and 1.1 to fetch resources. Due to their depreciation and security risks, these protocols have been disabled on all of our servers.
In order to address this, the code of the website has to be updated to utilize TSL protocol 1.2 or higher. For issues such as this one, you will have to consult/hire a professional developer to check and address it for you. If you do not have a developer in mind, you can check our suggested third-party services.