I had a similar issue with my HT802 when I had TLS enabled on the SIP account. Still haven't been able to get past that, so I had to set it to TCP on 5060. Had to reset it to defaults then reconfigure it as it would go into a reboot loop with the TLS enabled and I couldn't get back into the web interface.
If this isn't what's happening to you, I won't be of much help!