Since moving to the 6.3R4 code, our Network Connect users have not been able to connect to the SSL VPN Gateway with their AT&T Air Cards. To solve this issue, you need to turn off the web acceleration software feature on the AT&T Air Card. Once that is done, the Network Connect client works fine. We did not see this issue in 6.3R3 code. Hope this helps anyone who is having the same problem.