I recently installed 10.8.1 and now cannot add a field to any shapefile. I was able to add one to the first shapefile, but as soon as I tried to add the second I get an error messages that states 'failed to add field to the table/feature class'. I have tried other feature classes and tried exporting a feature class to a new one, no luck. I then uninstalled 10.8.1 and reinstalled and still have the same issue. Everything worked fine while I was using 10.4.