In /etc/X11/xdm/Xsession, the variable $XKB_IN_USE in tests at lines 55 and 66 should be enclosed in double quotes. Have hit cases where value is null, thereby causing syntactic error in script. Sorry, cannot read portion of my note about conditions that caused the variable to have null value.
*** Bug 17711 has been marked as a duplicate of this bug. ***
Fixed in rawhide (not sure when as I just found it). ;o) This is part of the xinitrc package.