it may not be metro, but, doubt the old WIn7 is at fault. Cannot really tell which process because it didn't specify the process name, or at least I didn't take time to read them. I recovered my Win7 yesterday, so, I cannot test it anymore.
Anyway, I have this problem 2 times. The last time I had lots of Metro apps opened and trying to log off, and after the error, I still have few Metro apps running. That's the only clue I have.