setting Reply URL

I have Core Web app in a Service Fabric service running, single node.  It uses AzureAD for authentication. This works fine, the AzureApp hashttps://localhost:12345/signin-oidc  as reply Url defined and the app is running on that port. 

Now we modified this service, allowing it to run on multiple nodes and without a fixed port number. So it is running on 5 nodes under different port numbers. And we do not really know up front what portnumbers. We have another node that acts as a gateway and it is running on and https://localhost:12345 and forwards the requests to the other 5 nodes.

The problem is that after the user logged in in Azure we get an error like 

So how can we set the Reply URL to a fixed value? 

I was expecting I could do something with the OnRedirectToIdentityProvider event, but it is never reached. 

Surprising as the documentation states 'Invoked before redirecting to the identity provider to authenticate', I would expect to hit it BEFORE I go to the login page.

Thanks

  Ben

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