Activity reports fail with "The Connection String has not been initialized" error (4286691)

When running any activity report (particularly the Activity by User and Activity by Document reports) that reads against SharePoint's activity data, it fails after 3-5 seconds with the error message below:

The Connection String has not been initialized

ControlPoint activity reporting reads against SharePoint's Logging and Web Analytics databases depending on the timeframe of the report. To do this, we utilize the Usage and Health Data Collection and Web Analytics service applications associated with the ControlPoint web application.

If no association to these service applications exists, the ControlPoint application cannot create a Connection String pointing back to the Logging or Web Analytics database.

1. Navigate to Central Administration > Application Management > Manage web applications.
2. Select the ControlPoint web application and click Manage > Service Connections.
3. Confirm that the web application is associated with the default service application group or confirm that the Usage and Health data collection and Web Analytics Service Application connections are selected such as the image below: