Here's what I have:
JSON Configuration -> oxTrust Configuration -> logoutRedirectUrl:
Those are the default values as far as I know. Yet, when I log out of oxTrust, I get the error message in my original post. This is not using OpenID, OAuth, SAML, or any other apps - this is just login/logout of the oxTrust management page with the admin account. It's certainly possible that I've changed something, but I've exhausted searching in logs and I'm stumped.