It can be a problem if a table is empty, an incorrect SQL query is used, an incorrect connection parameters are used and etc.

Try to set "Debug" parameter to "Yes" in DB connection settings ("Components -> ARI Smart Content -> Data -> Databases -> DB Connections" page) to see a reason of the problem.

PS: We can investigate the problem in more details if you provide a temporary access to your J! backend by email.

Regards,
ARI Soft