I tried just removing the existing config file (left the existing ui_config) and that also allowed ardour to start correctly.
I think the problem is that Ardour now supports multiple backends (direct ALSA connect and others, in addition to just JACK backend), and the config file should now contain the backend configuration. If there is no config file Ardour will prompt for the backend configuration when it starts, but if there is an existing config file with no backend configuration information it crashes.