I too had exactly the same error of course completion that you have posted here. After reading the messages posted, what i had done is to change the Grade Category Setting.
Go to Site administration ► Grades ► Grade category settings
In Aggregation (grade_aggregation) previous i had set it as Mean of Grades, which when i changed it to natural, had resolved this error and even cron worked after this setting has been changed. May be the system is not calculating the "Mean of Grades".
Also in the meanwhile, you can change the weightage for each of the course in "Gradebook Setup" under Course Administration and change the weightage of each course to tally a total of 100.
Im just one month old to Moodle. So still in exploration. If any other solutions works or worked for you, please let me also know.