15" MBP Mid-2009. Just upgraded from 10.8.4 to 10.9 Developer Preview. Installation went through without errors but upon first boot into the new OS, while Finder and startup programs are still loading, I get kicked right back to the login screen. I can enter my password and log in, but within a few seconds get kicked right back to the login page. I have a few seconds before getting sent back to the login screen, during which I am able to launch Finder or System Preferences via the Dock, but it's not long enough to actually do anything. I have tried Safe Boot, and the exact same behaviour occurs. Same thing happens if I login as "Guest User", so it's not just MY user profile. Where do I begin troubleshooting this?