I had this exact same problem. Are you based outside of the US? I'm from Singapore, and I've read about other Singaporeans having the same "Temporary session is invalid" problem. The workaround that worked for us is to access the forum using a US proxy. I used one from Washington D.C., and it allowed me to log in and register. After registration, the proxy is no longer needed to access the forums, so it seems like there's some kind of IP check for the first registration.