Are you trying to share a link that's protected by CBSubs? If so it's trying to display them the plan selection page to access that content and if they're not already logged in it asks them to register. You can override this redirect to whatever you like using "Membership needed for access redirect URL for existing users" parameter under CBSubs > Settings > Display > Expiration, but I don't recommend doing so. Having your login module present on your registration page either through a module position or setting "Show Login form next to Registration form" to "Yes" within CB > Configuration > Registration would probably be an easier solution.

Strange, seams to be working fine for me in my tests. You could have a CSS conflict issue with an element overlapping the login form or some sort of jQuery blocking it. Maybe try just using a standard module in a module position?

Disable the login module on registration within CB > Configuration > Registration then just have a regular login module that's set to be displayed on all pages.