This has provided us with ever increasing frustrations as it's taken us a long time to figure out where our clients are getting these incorrect URLs to login with. Essentially what is happening is SS is pulling the Network URL instead of the Site URL and when the clients are clicking it, WP isn't pushing through the redirect to the correct URL because the domains are different due to domain mapping.
This is what it's providing:
This is what it should be:
This is what WP is trying to redirect to because SS is pulling the Network URL which isn't correct:
This URL does not allow a user to login. It's stuck in a loop.
Ultimately this is solved by the plugin being reconfigured to pull the Site URL instead of the Network URL.