Even if you had setup nginx reverse proxy w/ssl (which would be an odd/redundant thing to do if you also have a HA Cloud subscription), you should still be able to connect directly to the HA server on port 8123 without ssl; the cert is only used by nginx not by HA itself.
Also if the cert was merely expired you would get a safety warning and be able to click-through to the site, so I would suggest that is not the root cause.
A few other people reported getting the same error as you when they switched from using HA Cloud to setting up a local nginx proxy — the app was still trying to use the previous settings, so maybe try removing the cloud option anywhere you can.
Also, if you know the Pi’s IP address, open that in a web browser, e.g. http://192.168.1.20:8123/