When a multisite is created and the SSL fails, if you visit the domain name, a 404 page is shown. This is because the site is never set up with the appropriate HTTPS configuration block in OLS. This leads to confusion for those that aren't technical at heart and more investigation and time is required to resolve the issue.
I suggest changing how sites are configured in GridPane for both OLS and Nginx. The HTTPS server configuration should be set up, and a self-signed SSL certificate should be put in place (the GridPane _default certificate is acceptable) when a new site is created. This would then result in an SSL error (Classic Your connection is not private) versus a 404. The SSL error can be bypassed, and the visitor can proceed to the configured site.
At this point, it's easy to identify that the site in question is missing an SSL certificate. If you have Cloudflare, you'll get an origin SSL certificate error. You can then enable flexible SSL, which disables SSL certificate verification, and the site will function.
This can come in handy, especially if you're struggling to get a certificate generated by letsencrypt and need access temporarily or want to use Cloudflare instead.