Carl:

Does the error occur on every single record?

When you created a new tax group and added it to your import file did you change every record to the new tax group, just some, or just one? In other words is the import complaining about the new tax group?

I just tested 3100 ship to by exporting and then importing. the import complained about some of the tax groups being invalid. When I investigated they were old tax groups that no longer existed.