10.4.4. Problems with the X Server Crashing and Non-Root Users
If you are having trouble with the X server crashing when anyone logs in, you may have a full file system (or, a lack of available hard drive space).
To verify that this is the problem you are experiencing, run the following command:
A key indicator is 100% full or a percentage above 90% or 95% on a partition. The
/tmp/ partitions can sometimes fill up quickly with user files. You can make some room on that partition by removing old files. After you free up some disk space, try running X as the user that was unsuccessful before.