Do you have TLS enabled? The error message “ERR_CERT_COMMON_NAME_INVALID” says, that the “common name” (it’s part of the “subject” extension) is invalid (I don’t know if “-200” is meant as an error code or the actual value of the “common name”, though). I would check the certificate and any attributes in there (you could post a redacted version, too).
Have you tried to open Mattermost in your browser on your Desktop PC? Or did you mean that by “mobile app”?
Maybe there’s more information that could help troubleshooting