It is important to make sure that your system has the correct time settings, both for making sure that your payments are recorded on the correct date, but also because if your time server is out of sync, some browsers may expire the cookie before the person is even able to start their reg process.
sudo apt-get install ntp
sudo joe /etc/ntp.conf
server time-a.nist.gov
server time-b.nist.gov
server time.nrc.ca
sudo /etc/init.d/ntp restart