This error can occur if at least one of the table links is using a different join type than the rest.
  1. Select Database > Database Expert
  2. Click on the Links tab (maximize the Database Expert window if necessary)
  3. Check each link by double-clicking on the black line between every table to make sure they are using the same Join Type (Left Outer Join is recommended)
  4. Click OK and refresh the report
Please note that in general all join types should be Left Outer Join, however in certain cases (when the data in the tables must match exactly) the join type may need to be Inner Join.