Hi Louis, I've also responded to your support call on this issue, copying my answer here for the benefit of the forum.
The Deeplink module uses its own login page by default, since the "regular" Mendix login systems won't redirect the user back to the deeplink. The page that is used for this can be found in the "resources" folder in your project directory: resources\deeplink\login.html. The styling of this page can be edited to conform to the rest of your application. Unfortunately, the current version of this file is still intended for Mendix 4.x, and doesn't pick up the new styling of Mendix 5. We will update the next release of the deeplink module in the appstore with a new login.html file that works with the default Mendix 5 styling.
The login location constant you mention should not be used for this. This constant is intended for use with single-sign-on solutions, where the user is redirected to a different URL to log in, and then back to the original deeplink once a session is established. As said, the default Mendix sign-on pages do not support this redirection (this is intended), so the deeplink module uses its own login page. If you are not using any special login functionality (such as SSO), leave this constant empty.
1) Did you debug your WF ? as it seems you run into a loop?! 2) Are you Emulating Cloud Security ?