Edit exported file and enter letter grades using the spacebar to terminate typing of a letter grade (needed to cancel autocompletion in excel) - the grades are not allowed on input.

When typing grades into a csv or xls or xlsx file, and using a space following a typed grade (e.g. to cancel excel autocompletion) the resulting saved gradefile evidently includes the space in the value uploaded. The gradebook import code does not trim trailing (leading?) blanks so it doesn't recognize that a "b " is the same as a "B" or a "b".