Comment 5 for bug 1451613

The same problem has been reported as questions in AskUbuntu:

http://askubuntu.com/questions/618189/computer-logs-out-while-i-lock-screen
http://askubuntu.com/questions/611717/ubuntu-14-04-logs-me-out-when-i-try-to-lock-screen

I'll point these people here, but it seems like this is a pretty widespread problem.