Try each of the following steps in order until the error is resolved:
 
  1. Ensure the export is using the correct query
     
  2. Check the query sort order to make sure it is correct
     
  3. Re-export the query creating a new file not overwriting the existing file
  4. If exporting in MS Access Report Writer Database format from The Raiser's Edge 6:
    1. Open the RE.INI
    2. Find [GENERAL] and add the following setting in this section:

      queryorder=1

      This forces the Export to use the order of the query if exporting in MS Access Format.
  5. Sort the export by a one-to-one field