We have also seen this after I upgraded to Version 3.5.5 on the CAS and CAM.
 While most of the time, this seems to be something on the user's systems
(firewall, multiple interfaces, illegal DHCP servers, etc) interfering with
communications to the server, this has not resolved all of our complaints.

There are a number of cases where the firewall is shut off, but the login
remains grayed out.  I put one of these systems on a hub and watched the
traffic from the system and never saw a single UDP packet transmitted from
the Agent (v3.5.6) when the system was rebooted and logged into.  I
uninstalled 3.5.6 and reinstalled 3.5.4, but still saw the same behavior, or
lack thereof.

Has anyone else seen this or know of other potential checks?

-Bill
Network Security Administrator
Housing Technology Services
Colorado State University