a11y bus trouble after upgrade

I have been upgrading eg dbus, glib, at-spi2-core etc. and now registryd
won't start. The best test so far, which is I think what
get_accessibility_bus_address_dbus() does in at-spi2-core atspi-misc.c is:

  dbus-send --print-reply --dest=org.a11y.Bus /org/a11y/bus \
  org.a11y.Bus.GetAddress

which returns:

  Error org.freedesktop.DBus.Error.Spawn.ChildExited: Process org.a11y.Bus exited with status 1

This suggests that the session bus is running sufficiently to accept a
connection, but then something fails - any clues on how to find out what?

Cheers,

Patrick