I lost many hours while setting up some self-signed certificates; the problem turned out to be that the browser (Chrome) cached a bad version of the certificate and then just wouldn't let go. I did lots of experimentation modifying the certificate, only to learn these experiments were testing nothing because the browser was not reading the new versions of the certificate.
Eventually, I clicked on the "not valid" indicator which shows up in the browser's address bar, just to the left of the URL. Who knew that this was an active element? Clicking on it allowed me to see browser's version of the certificate. The expiration date and time didn't match the newest certificate; that's how I knew the browser was using the wrong version.
I found another, similar question on StackExchange, but none of the answers worked for me. (The Chrome "settings" interface keeps changing.) So what does work on Chrome version 81, to clear the certificate cache?