It's possible a user of SFMC Quick Search is getting one of the following (or similar) issues when trying to use the search functionality:
"Error: 401 Unauthorized"
To avoid this kind of error when performing searching in SFMC Quick Search, make sure your users have the permissions listed in this article: Required Permissions for SFMC Quick Search users.
Also, make sure the Marketing Cloud Subdomain is filled out correctly. Click on Need Help? underneath the Marketing Cloud Subdomain input for details on what to fill out.