Downgrade glib2 to 2.74.6 until the stuff it breaks is fixed
Search the Arch forums for glib2. Start with https://bbs.archlinux.org/viewtopic.php?id=284268.
glib2-2.75.4 also has the bug (technically it's not a bug in glib2 but a laziness in prgramming that it no longer permits; and of course they didn't warn anybody of the change).
As it turns out, I can trigger the bug in openbox with glib2-2.74.6 by starting it with
Exec=sh -c "G_SLICE=always-malloc openbox"
in ~/.config/autostart/openbox.desktop. It seems that with glib2-2.76.0 G_SLICE defaults to always-malloc.