You receive an error saying a field should not be null when running Duplicate Remover. This is typically caused by a true/false field with nulls. We see this fairly often, particularly on ACT! data bases that started back around ACT! 2006 or 2007 and have since been upgraded to newer versions. Several records have NULL in a YesNo field. You can prove this on your data base by doing a Lookup/All Contacts to get a total record count. Then Lookup/Field/Equals/True and note the count and then Lookup/Field/Equal/False and note the count and you will see that the True count plus the False count is less than the total, indicating how may records you have with nulls. This was a bug with YesNo fields back before about ACT! 2008 or 2009.
We use a long process to fix a YesNo field (it involves defining a new YesNo field, doing a Lookup for True in the old field and then an Edit/Replace with True in the new field then Edit/Copy of the New field back into the Old field for all records) that can take a very long time on a large data base and ends up changing the Last Edit Date of every record to Today and this may not be what you want in this situation.
Our most recent versions of Duplicate Remover Expert detect the issue when it is combining records and force a set of the Keeper record YesNo field so that the Keeper record can't end up with a Null in it. Either run the solution as noted above or contact us to get the most current version of your product.