Commenting out
db_unregister xserver-xfree86/manage_config_with_debconf
db_unregister xserver-xfree86/move_existing_nondebconf_config
at the bottom of the preinst script seemed to work around the problem.
At first, the installation aborted with a segfault in the postinst,
but another --configure completed, and the X server is now running.