Resolve Web Site Login error in the Sandbox Web

This solution explains why Customers are unable to login as Shoppers to their Sandbox Web Site, especially after it has been refreshed.

This is because during refresh, the Customer Center role assignments for Customer users are not copied from Production to Sandbox Accounts.

Note: More details can be found in the SuiteAnswers article Features Available for Testing in a Sandbox (Answer ID: 19470).

Solution:
In order to provide access back, remove the Customer Center Role from the Customer record and re-assign it again.