Sorry, that version was end-of-life over 6 years ago. The error does not sound familiar either, unfortunately.
The certificate should not be an issue (i ran my old 6.x with expired certs for years).
That said, as i recall from the old days....... try removing the firewall from the console and re-adding it. Optionally, you can try updating the certificate (but i dont think thats the issue here)