We experienced this issue, but in our case it wasn't an error, it seems to be a feature .
When a teacher gives a feedback to a student in a particular quiz, the grade automatically get blocked even when the quiz still open.
The teacher gave feedback to all students, those who had tried the quiz and those who didn't have tried and this action made the grade get blocked, the Override check box automatically check.
The only thing I have to do to recalculate the grade is uncheck this option by activity in the edit icon on the corresponding columns of the activities in the grader report. Or telling the teacher not to fill a feedback
I hope this helps to someone.