Essentially logging out a user under yosemite "kills" parallels and it's VM. When you login that user again parallels and the VM (XP) will not start. It's get's a "critical error". Rebooting the MacBook clears the problem. If I shut down the VM and stop parallels and then logout the user everything is fine. Tried both scenarios many times with the same results every time. When you "logout" the user under Yosemite it is GONE in less than 2 seconds. Suspect it is just issuing a kill. Looked for some configuration options but cannot find any. I don't know what the behavior was before parallels 10 and yosemite. Never tried to run multiple users.