Answer: When test databases are created, the connection to the User-Defined Reports module can be compromised. In order to get your report running in the test environment, try republishing it.
To do so, navigate to User-Defined Reports in your UAT database and choose your report from the EXISTING REPORTS drop down. Click on the TITLE tab and click PUBLISH. This will refresh the connection to the report in your test environment.
Please don't hesitate to contact Support if you continue to experience issues.