Machines appear and disappear in WLK 1.2

Latest WLK, I'm having some issues with machines that appear and disappear
from the controller. The logs have some errors such as:
1844 1952 2008:6:15 15:39:32:968 Error: 0x80040e14, Error 0x80040e14
(Error hr: 80040E14 (-2147217900) from call: pADOCommand->Execute (NULL,
NULL, 0, &pADORecordset) )
File=d:\3360\sdktools\wtt\jobs\runtime\common\db\s rc\wttdbaccess.cpp
Line=1221 WINXP-VM13->CONTROLLER:
1844 1952 2008:6:15 15:39:32:968 Error: 0x80040e14, Error 0x80040e14
CMGMachineIDMessageHandler:Unable to Execute Query:"EXEC
[dbo].IdentifyResource_SP N'',
....

Is it some database corruption?

The machines are VERY similar to one another - what makes a machine unique
to the controller (I suspect a combination of several attributes, just not
sure which).