This is a bug that has been reported by others, perhaps in different variations from your setup, but one that is fairly common. I see it with Vista, Win7 and XP on occasion on some of my PCs, so find myself reluctant to use the Live Mesh RD these days! When I discover the blank desktop that I can't access, I need to remote in again and reboot to resolve it.
I suggest that you file a bug report with your specific details:
How to Submit Bugs and Live Mesh Logs
Note that you may not receive a response to the bug report, though. It will be transferred to the internal system to be assigned to the appropriate development group.
-steve
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