X server not starting on boot

[Raman Gupta]

My X server is no longer starting at boot time. Plymouth simply stops
and X does not start.

I use the open source ATI radeon drivers on a fully updated Fedora 21
system. systemctl status kdm.service reports that the X server failed
to start:

Jan 16 13:16:02 host kdm[1885]: X server for display :0 terminated
unexpectedly

But /var/log/Xorg.0.log contains absolutely no errors, except that
where the following lines would normally be printed:

[   171.281] (II) RADEON(0): EDID vendor "NEC", prod id 26260
[   171.282] (II) RADEON(0): Using EDID range info for horizontal sync
[   171.282] (II) RADEON(0): Using EDID range info for vertical refresh
[   171.282] (II) RADEON(0): Printing DDC gathered Modelines:
[   171.282] (II) RADEON(0): Modeline "1680x1050"x0.0  146.25  1680
1960 2136 2240  1050 1053 1059 1089 -hsync +vsync (65.3 kHz eP)
...

instead it looks like it just starts shutting down:

[    24.280] (II) UnloadModule: "wacom"
[    24.280] (II) evdev: Eee PC WMI hotkeys: Close
[    24.280] (II) UnloadModule: "evdev"
... [ more close / unloads, until log end ]

I am able to start X by ssh'ing in to the box (no VTs are available
either), and restarting the kdm service manually (sudo systemctl
restart kdm), at which point everything works perfectly well.

Any ideas what is going on here?

Regards,
Raman