I thought this might be related to time so I checked the config on the firewall and DC and everything was in perfect sync, so I don't think that is the issue. I would just try and try and eventually it would work. I have had some seemingly random success. I have had some success with a simple reboot: close NetExtender, Reboot. I wish I had a more complete answer for you, but I have the same problem, and it's not 100% resolved.
