Symptoms

After upgrading an environment, there was some trouble with SmartObjects. When trying to execute a SmartObject, an error was being displayed, reading:

"Root Element Is Missing"

This issue is happening across all SmartObjects, though deleting them and remaking them with the same GUID fixes the issue. These SmartObjects are based off of SQL tables.

 

Diagnoses

The SQL Service Instance is pointing to the wrong database.
 

Resolution 
We edited the SQL service instance and pointed it to the correct database. After that, the smartobjects returned all information correctly.