I found this on another post, this may be the problem, we are trying the fix now to see if it works:
"That issue is caused by Microsoft's Internet Explorer browser going into Work Offline Mode. It's kind of a bug w/ the pre 4.8 releases of NAC, though not publicly communicated. The NAC agent is using an IE API to perform it's SSL work and if IE is open and trying to refresh during the login process, then IE goes into offline mode automtically. The workaround is to have the user disable offline mode in IE, and close out teh NAC error message. It typically fixes it within a few seconds and NAC authenticates normally."
We are running the 4.8 client, so it looks like the client bug has not been fixed yet.