We had a course that the teacher entered grades for an exam, and it displayed "ERROR" for all of the grades for that exam. I went to look at the gradebook history, and sure enough, grades were entered. I went to the grader report, turned editing on, and then tried to lock and unlock the column (with no success). Then I clicked on the little wheel to edit that column, made no changes, but clicked "Save Changes". It then recalculated the grades, and the ERROR went away for all of the grades.
Strange behavior. Glad it got fixed, but hope it doesn't happen again!
Moodle 3.5.1+ (Build: 20180831)