I have had the same problem on a desktop-pc Dell GX755
The only cure I have found for both of my usb key-b and mouse is to unplug them and plug them back on.
The same problem exists in slim, lightdm, lxdm, so it is not dm related. They both freeze up while on the login screen (console keyboards works fine). After plugging them back on everything runs fine till next reboot. Pulling one and not the other seems
to have independent effects. Mouse gets freed up while keyboard is locked and vise versa.
I am hoping this fix will give the developers a clue of what may be causing it.
X0rg.log provides nothing significant as mentioned before. I assume it would only report a problem if it identifies one.