There is a limit of 3 sysprep runs from a single OS install. It is true that you are limited to three but there is an easy workaround. The limit manifests itself with another fatal error during sysprep and the following message in the log file:
RunExternalDlls:Not running DLLs; either the machine is in an invalid state or we couldn't update the recorded state, dwRet = 0x1f
The work around is to set the following reg keys:
HKEY_LOCAL_MACHINE\SYSTEM\Setup\Status\SysprepStatus\GeneralizationState\CleanupState: 2GeneralizationState: 7 (doesn’t have to be 7 but that’s what this guy did)
Reboot, magic.