Systems sudenly dropping out of SCCM

On the SCCM Console, under 'Client'=No for several active workstations that had reported being clients fine a few weeks ago.  If I go to the systems itself, the client is install normally and appears to be working fine even though I cannot manage it from the console.  One the affected clients,  I'm seeing one thing in common is in execmgr.log:

Failed to instantiate UI Server {C2F23AE4-82D8-456F-A4AF-A2655D8CA726} with error 8000401a      execmgr      7/28/2011 5:00:47 PM      4488 (0x1188)
Failed to instantiate UI Server 2 {E8425D59-451B-4978-A2AB-641470EB7C02} with error 8000401a      execmgr      7/28/2011 5:00:47 PM      4488 (0x1188)
Failed to instantiate Updates UI Server {2D023958-73D0-4542-8AD6-9A507364F70E} with error 8000401a      execmgr      7/28/2011 5:00:47 PM      4488 (0x1188)
Failed to instantiate VApp UI Server {00AAB372-0D6D-4976-B5F5-9BC7605E30BB} with error 0x8000401A      execmgr      7/28/2011 5:00:47 PM      4488 (0x1188)

Anyone have an idea of why all of a sudden they would drop out?  I do not see any other errors in CAS.log, ClientLocation.log, ClientIDManagerStartup.log or CcmExec.log  I also verified Site Maintenance Tasks to ensure no systems are being purged out in error.