My Ubuntu 12.04 box has been working fine. A recent update may have messed something up.

I normally run remote windows on it, and I noticed that my windows were failing to start up. I then tried logging into it directly from the GUI console, and I'm seeing that after I press enter on the (valid) password, the page just redisplays. It's not a password error, as that would give me an inline error. I see some messages appear and disappear quickly between the login screen going away and then redisplaying, but they go away too quickly to read.

I was able to run the non-gui login, and I did an update and upgrade, and then rebooted, but it's doing the same thing.

I have a Samba connection from my Windows box, and that's still working.

If it matters, here's my uname output (somewhat elided):

Linux ... 3.2.0-26-generic #41-Ubuntu SMP Thu Jun 14 17:49:24 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

What can I do to troubleshoot this?

Note that when I select "Guest Session", it lets me log in and displays the window manager. This seems significant to me. Does this mean that something specific to my login is causing it to fail?

Note: If it matters, here's the output from /var/log/dmesg. The line about gdm seems interesting:

[    9.815883] Bluetooth: RFCOMM TTY layer initialized
[    9.815887] Bluetooth: RFCOMM socket layer initialized
[    9.815888] Bluetooth: RFCOMM ver 1.11
[    9.879088]  [PCSPP,TRISTATE]
[    9.879092] parport0: irq 7 detected
[    9.883935] type=1400 audit(1341871177.871:10): apparmor="STATUS" operation="profile_load" name="/usr/lib/lightdm/lightdm/lightdm-guest-session-wrapper" pid=845 comm="apparmor_parser"
[    9.884365] type=1400 audit(1341871177.871:11): apparmor="STATUS" operation="profile_replace" name="/usr/sbin/ntpd" pid=851 comm="apparmor_parser"
[    9.950397] e1000e 0000:00:19.0: irq 42 for MSI/MSI-X
[    9.961160] init: gdm main process (907) killed by TERM signal
[    9.966358] lp0: using parport0 (polling).