The problem: when your new student signs up and receives their membership email, the button in this email has an incorrect link.
To update the link to this button, you need to add the correct redirect page link for the Member Group on your CCK dashboard.
If your redirection URL is incorrect when a student buys your course, the email they receive will also have an incorrect login link.
Once an email has been sent to a student, the link in the email will remain incorrect. You can not resend the system email. The only way to resend this email is if a student goes through the checkout process again.
We recommend that you sign up for your own course as a student with a 100% (free) discount code to test this email and check that you have set up your links and course details correctly before you send any of your students through your checkout process.