The workgroup you are working with has probably become unusable and cannot be fixed. One option is to duplicate the workgroup by clicking the Duplicate button. The duplicate can now be edited, so you can delete the original and use the duplicate.
If you are unable to duplicate or delete the workgroup, you can force At Ease to run a Consistency Check Report. To do this, quit the At Ease Administrator application. From the Finder, open At Ease Administrator again, this time holding down the Command key. Continue pressing the Command key until you are asked for the Administrator's password. Release the Command key to type in the password, and then press the Command key again before you select the "Okay" button. When you get the next dialog box about validating the users and workgroups databases, release the Command key and select "Yes" to let the Consistency Check run. This will check for any los in the databases and At Ease will fix the issues if it can.
Another option would be to delete the workgroup which is getting errors, and build a new workgroup.