Overview

When trying to search emails, no one can search for emails and the error 'Search failed with an unknown error' is seen.

Solution 

The most probable cause for this error is due to invalid Index File Location Credentials which occurs if the Administrator password was changed.

Follow the steps mentioned in the article for Configuring Index File Location Credentials.

Testing

Access the Index file Location with your admin credentials to verify that the configuration of the credentials was successful.