Sounds like a permissions problem of some sort. Try again, and when it fails, use your text editor of choice and look at .xsession-errors in your home directory. That may tell you where you're locked out.
The correct solution depends on your distro. Since I don't know what distro you're running, I don't know what the correct solution is. You can, of course, cheat by making the program you're locked out of run suid root, but that causes security problems.