This is generally due to database corruption. Our recommendation is to first attempt to remove the corruption from your database file and if that fails to resolve the issue, restore a recent backup that you made prior to this error occurring.
Click here to run the database maintenance utility to clean your database file.
Click here for the steps to restore a backup.
If the issue persists after restoring a backup, go to www.gradiencesupport.com and click on Submit a Request.