This is affecting both rawhide composes and f28 updates (for building atomichost installer images). 

See: https://koji.fedoraproject.org/koji/taskinfo?taskID=27136573
and from it's root.log: 

DEBUG util.py:439:  running runtime-postinstall.tmpl
DEBUG util.py:439:  2018-05-23 07:31:45,827: template command error in runtime-postinstall.tmpl:
DEBUG util.py:439:  template command error in runtime-postinstall.tmpl:
DEBUG util.py:439:  2018-05-23 07:31:45,827:   gconfset /desktop/gnome/interface/accessibility bool true
DEBUG util.py:439:    gconfset /desktop/gnome/interface/accessibility bool true
DEBUG util.py:439:  2018-05-23 07:31:45,829:   subprocess.CalledProcessError: Command '['gconftool-2', '--direct', '--config-source=xml:readwrite:/var/tmp/lorax.sonh521w/installtree/etc/gconf/gconf.xml.defaults', '--set', '--type', 'bool', '/desktop/gnome/interface/accessibility', 'true']' died with <Signals.SIGABRT: 6>.
DEBUG util.py:439:    subprocess.CalledProcessError: Command '['gconftool-2', '--direct', '--config-source=xml:readwrite:/var/tmp/lorax.sonh521w/installtree/etc/gconf/gconf.xml.defaults', '--set', '--type', 'bool', '/desktop/gnome/interface/accessibility', 'true']' died with <Signals.SIGABRT: 6>.
DEBUG util.py:439:  Traceback (most recent call last):
DEBUG util.py:439:    File "/usr/sbin/lorax", line 285, in <module>
DEBUG util.py:439:      main()
DEBUG util.py:439:    File "/usr/sbin/lorax", line 141, in main
DEBUG util.py:439:      remove_temp=True, verify=opts.verify)
DEBUG util.py:439:    File "/usr/lib/python3.6/site-packages/pylorax/__init__.py", line 294, in run
DEBUG util.py:439:      rb.postinstall()
DEBUG util.py:439:    File "/usr/lib/python3.6/site-packages/pylorax/treebuilder.py", line 146, in postinstall
DEBUG util.py:439:      self._runner.run("runtime-postinstall.tmpl", configdir=configdir_path)
DEBUG util.py:439:    File "/usr/lib/python3.6/site-packages/pylorax/ltmpl.py", line 220, in run
DEBUG util.py:439:      self._run(commands)
DEBUG util.py:439:    File "/usr/lib/python3.6/site-packages/pylorax/ltmpl.py", line 239, in _run
DEBUG util.py:439:      f(*args)
DEBUG util.py:439:    File "/usr/lib/python3.6/site-packages/pylorax/ltmpl.py", line 476, in gconfset
DEBUG util.py:439:      runcmd(cmd)
DEBUG util.py:439:    File "/usr/lib/python3.6/site-packages/pylorax/executils.py", line 341, in runcmd
DEBUG util.py:439:      return execWithRedirect(cmd[0], cmd[1:], **kwargs)
DEBUG util.py:439:    File "/usr/lib/python3.6/site-packages/pylorax/executils.py", line 228, in execWithRedirect
DEBUG util.py:439:      env_add=env_add, reset_handlers=reset_handlers, reset_lang=reset_lang)[0]
DEBUG util.py:439:    File "/usr/lib/python3.6/site-packages/pylorax/executils.py", line 201, in _run_program
DEBUG util.py:439:      raise subprocess.CalledProcessError(proc.returncode, argv, output)
DEBUG util.py:439:  subprocess.CalledProcessError: Command '['gconftool-2', '--direct', '--config-source=xml:readwrite:/var/tmp/lorax.sonh521w/installtree/etc/gconf/gconf.xml.defaults', '--set', '--type', 'bool', '/desktop/gnome/interface/accessibility', 'true']' died with <Signals.SIGABRT: 6>.

This is because GConf2 is no longer installed by default... 

Running in a chroot I see: 

# chroot /var/lib/mock/f28-build-12504221-915238/root
bash-4.4# gconftool-2 --direct --config-source=xml:readwrite:/var/tmp/lorax.mqeila51/installtree/etc/gconf/gconf.xml.defaults --set --type bool /desktop/gnome/interface/accessibility true

(gconftool-2:5532): GConf-WARNING **: 17:54:27.261: Failed to load source "xml:readwrite:/var/tmp/lorax.mqeila51/installtree/etc/gconf/gconf.xml.defaults": Failed: Could not make directory `/var/tmp/lorax.mqeila51/installtree/etc/gconf/gconf.xml.defaults': No such file or directory
**
GConf:ERROR:gconftool.c:969:main: assertion failed: (err == NULL)
Aborted (core dumped)

So, we should get lorax to no longer use gconftool-2. Perhaps dconf?