When taking a break from working in my laptop (Xubuntu 14.10), I always lock the screen on my pc, keeping many applications running. Usually all is normal, regardless of the idle time I take. However, this is the second time that I lose my session after an idle time with the X session locked. It happens like this: when I return to my station, I log in, and then, after a few weird flashes, it goes back to the main log in screen again (just like if I had restarted the computer, which is not what is happening by the way because I have a boot password that prevents the immediate run of the OS). When I enter my password again, a new session starts, (no previous application running). However, there is some memory as the browser remember my last tabs open and gmusicbrowser's queue list.
Both times that I have lost my session have been long idle times (more than one hour). Apparently, this only affects me if idle time is long. I tried to reproduce the problem by short idle times and I can't. I will try to reproduce it for lon idle times but that is not very handy.