Unable to redirect to external URL from Java

Hi,

I have an app with that uses the Open Authentication module (https://github.com/Erwin-t-Hoen/Open-Authentication-Module) to implement Facebook login functionality.

While the app was running in a sandbox, this functionality worked without any issues (even on the same SVN revision). The app was recently moved to a Azure Windows Server 2016 virtual machine. The server was configured according to documentation (https://docs.mendix.com/howto/on-premises-deployment/deploy-mendix-on-microsoft-windows) and additional request handlers required by the module are configured as well (signin/, callback/, logout/).

Unfortunately, the redirect to Facebook is not working. Somehow, a redirect to Facebook will end up redirecting to the application root URL. This is the code in question, from oauthmodule\actions\custom\GetAccessCodeFacebook:

public class GetAccessCodeFacebook {

	private final String OAUTHURI = Constants.getOAuthURI_Facebook();
	private final String CLIENTID = Constants.getClientId_Facebook();
	private final String CALLBACKURI = Constants.getCallbackURI_Facebook();

	protected void getCode(String UUIDstate, HttpServletResponse servletResponse) throws IOException{
	Core.getLogger("OAuthSignin").trace("Get token from Facebook");
	StringBuilder oauthUrl = new StringBuilder()
	.append(OAUTHURI)
	.append("?client_id=").append(CLIENTID) // the client id from the api console registration
	.append("&redirect_uri=").append(CALLBACKURI) // the servlet that linkedin redirects to after authorization
	.append("&scope=email") // scope is the api permissions we are requesting
	.append("&state="+UUIDstate);
	Core.getLogger("OAuthSignin").trace("Url used for facebook: \n"+oauthUrl.toString());
	servletResponse.sendRedirect(oauthUrl.toString());
	}
}

The log in the second last line of code will output the following in the server console:

Url used for facebook: 
https://www.facebook.com/v2.8/dialog/oauth?client_id=414691225590004&redirect_uri=http://myapp.cloudapp.net/callback/facebook&scope=email&state=dc413975-18d5-4c93-bf09-71e2ef134bbc

however, when servletResponse.sendRedirect is executed, the user will be redirected to the following URL: 
http://myapp.cloudapp.net/v2.8/dialog/oauth?client_id=414691225590004&redirect_uri=http://myapp.cloudapp.net/callback/facebook&scope=email&state=dc413975-18d5-4c93-bf09-71e2ef134bbc

I tried changing the code to:

servletResponse.sendRedirect("https://www.facebook.com");

but this will again redirect me to: http://myapp.cloudapp.net

I looks like I am overlooking something simple here, I just don't see it. If I go back to the sandbox app, the redirect will still work as expected but it will somehow not work on my Windows Server. What are my missing? Help would be greatly appreciated. Thanks!

* The actual Mendix version of this app is 7.5 but I couldn't find it in the model version dropdown