I've sent this before to the Red hat forum, it got flooded out I belive, so I'm going to try here.

One day when I was turning on my computer, all was well until I pressed enter after typing my password. Then I get a window saying "This X session lasted less than 10 seconds"

The error output is

I've tried to install all the latest updates via yum, since I'm only in init 3. I can start the X server through the startx command as root, and as regular user, but the regular user doesn't get any permissions.

in the error window, I get it also says that there is no user profile for my user in the acl, I'm having troubles getting that window up now, so the error code you see is just the error output from nautilus.

I hope to see some answers

with thanks in advance :*

Could you check if everything works without SELinux set on enforce.
This will make SELinux useless, so make sure you enable it again and change the security context with chcon (see "info chcon") if it worked.