If for some reason the webgui certificate is broken, you can reconfigure access using the console menu. Select Set interface IP address (option 2) from the menu, reconfigure an interface, after providing the address configuration you can either (temporary) switch back to HTTP or in the next step generate a new self-signed certificate.
It is also possible to reset the defaults in the final step (“Restore web GUI access defaults?”), in case something went wrong while setting up anti lockout policies or after changing interfaces.