We continually get these errors since Moodle 2.8:
The error is (translated from Dutch): the grades are not being saved because someone has edited one or more grades since you loaded the page
Also a big problem is that the text is green, indicating that everything went well. But grades and feedback are NOT stored. The error should be red.
We are certain that this notification is not correct as we can generated it every single time when we try to add grades/feedback to the grade book.