Hah, I hadn't yet fixed anything, but had determined that this was an auth0 bug and was writing the support ticket showing the debug stuff... and they've fixed it.
Indeed. At the last attempt I got a plain 500 error rather than a lfgss error page, but at least the cookie had been created and going home showed me logged in.
Hah, I hadn't yet fixed anything, but had determined that this was an auth0 bug and was writing the support ticket showing the debug stuff... and they've fixed it.
Apologies on their behalf for the downtime.