The "system is misconfigured" error when a user hits the captive portal is an indication that the EAC appliance cannot determine which end system is attempting registration, or the end system is in a state in which it is not eligible for registration.
When the end system encounters the issue find the end system in NAC manager and make sure the following information is correct:
1. Has the NAC completed IP resolution successfully, is the IP address shown the real IP address of the client.
2. Is the end system in an "Active" state due to a RADIUS authentication and not other XML type event (Eg Identifi messaging)
3. Does the end system have a profile that was obtained through the "Unregistered" rule in NAC Manager.
Chances are that one of the above 3 conditions are not met. If the NAC doesn't have the correct IP address, or any IP address it cannot determine which end system is attempting to register and will error out.
If the NAC doesn't believe the end system should be registering (not in an active state, or not in the unregistered rule) it will error out and show you the error.