404 error on oauth authz endpoint

We upgraded from 4.5.3.3 to 5.0.1.1 and now are getting a 404 error on the authz endpoint with our apps that use .net 4.6 and Microsoft.Owin.Security.OpenIdConnect.  All of our other oauth clients work fine.  The client makes a request like this and NAM returns the 404 error:

https://xxx.example.com/nidp/oauth/nam/authz?client_id=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx&redirect_uri=https%3A%2F%2Fyyyyy.example.com%2Fcallback&response_type=code%2Bid_token%2Btoken&scope=openid%2Bprofile%2Bemail&state=OpenIdConnect.AuthenticationProperties%3DCo3T2iBIsJ5N6k6JulAxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxYhHYM8rJNADVv5kwayXhH9YPt0ncRCLitxvvqMGIeqn1iEEdfJpoO1JncRv7-0&response_mode=form_post&nonce=637xxxxxxxxxxxxxxxxxxxxxxxxxiLWEwMjMtMTJmYTVhYWUyYjZiMTZmOWFkNWYtNTQ5MS00NGZkLThhMDgtMGM0MDkzY2Y5YTQw&x-client-SKU=ID_NET461&x-client-ver=5.3.0.0

I have a case opened on the issue.  It sounds like someone had a similar case open before, but I haven't heard back on it yet.

Has anyone run into this?

Thanks,
Jeremiah