The ERR_SSL_PROTOCOL_ERROR is commonly related to an issue with your SSL certificate. If you have a Web or Cloud hosting plan and your website doesn't have an SSL certificate installed yet, you can easily install it for free.
If there is an SSL certificate installed on your website and you are still getting this error, you can try the following:
Clear your browser history and cache, or check the website using the incognito mode
You can also check the following video tutorial for more alternatives: