Overview:
The Approved Email Unsubscribe Page link generates a blank (400) page in a refreshed CRM Sandbox.
Root Cause:
The Email Record Unsubscribe Token is generated with a link pointing to the old ORG ID.
Solution:
Create a new Sent Email to generate a URL pointing to the New ORG ID.
Related Documentation:
CRM Help Documentation: Embedding the Unsubscribe Page